$rails testができなかった問題→rexmlを追加すれば良い

#問題
``$rails test``をしたらエラー文が吐かれた
ロードができないようだ

```terminal
sample_app $ rails test
Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:34:in `require': cannot load such file -- rexml/document (LoadError)
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:332:in `block in require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:299:in `load_dependency'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:332:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/selenium-webdriver-3.142.7/lib/selenium/webdriver/firefox.rb:22:in `<main>'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `block in require_with_bootsnap_lfi'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/loaded_features_index.rb:92:in `register'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:22:in `require_with_bootsnap_lfi'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:31:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:332:in `block in require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:299:in `load_dependency'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:332:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/webdrivers-4.5.0/lib/webdrivers/geckodriver.rb:72:in `<main>'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `block in require_with_bootsnap_lfi'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/loaded_features_index.rb:92:in `register'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:22:in `require_with_bootsnap_lfi'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:31:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:332:in `block in require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:299:in `load_dependency'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/activesupport-6.1.1/lib/active_support/dependencies.rb:332:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/webdrivers-4.5.0/lib/webdrivers.rb:4:in `<main>'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `block in require_with_bootsnap_lfi'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/loaded_features_index.rb:92:in `register'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:22:in `require_with_bootsnap_lfi'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bootsnap-1.7.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:31:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bundler-2.2.6/lib/bundler/runtime.rb:66:in `block (2 levels) in require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bundler-2.2.6/lib/bundler/runtime.rb:61:in `each'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bundler-2.2.6/lib/bundler/runtime.rb:61:in `block in require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bundler-2.2.6/lib/bundler/runtime.rb:50:in `each'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bundler-2.2.6/lib/bundler/runtime.rb:50:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/bundler-2.2.6/lib/bundler.rb:173:in `require'
	from /Users/yuya/environment/sample_app/config/application.rb:7:in `<top (required)>'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application.rb:92:in `require'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application.rb:92:in `preload'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application.rb:157:in `serve'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application.rb:145:in `block in run'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application.rb:139:in `loop'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application.rb:139:in `run'
	from /Users/yuya/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/spring-2.1.1/lib/spring/application/boot.rb:19:in `<top (required)>'
	from <internal:/Users/yuya/.rbenv/versions/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/Users/yuya/.rbenv/versions/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from -e:1:in `<main>'
```

#原因
[Rails 6.1: What is preventing tests from running?](https://stackoverflow.com/questions/65479863/rails-6-1-what-is-preventing-tests-from-running)
こちらの記事を見たところ`rexml` gemを追加してないことによって起きたエラーだそうです

``rexml`` gem is a bundled gem since Ruby 3.0.0. 
(`` rexml`` gemは、Ruby3.0.0以降にバンドルされたgemです。)

So you must add ``rexml`` to ``Gemfile``.
(したがって、`rexml`を`Gemfile`に追加する必要があります。)

#解決
``rexml``を`Gemfile`に追加

```ruby:Gemfile
source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '3.0.0'

gem 'rails', '~> 6.1.1'
gem 'sqlite3', '~> 1.4'

/省略/

gem 'rexml'
```
`gem`を追加したので`bundle`

```:terminal
$ bundle install
```
#結果
`$rails test`ができました

```terminal:terminal
$ rails test
Running via Spring preloader in process 7862
Run options: --seed 12855

# Running:

....

Finished in 4.874390s, 0.8206 runs/s, 0.8206 assertions/s.
4 runs, 4 assertions, 0 failures, 0 errors, 0 skips

```