test_ruby_mode.rb: trivial refinement

* test/misc/test_ruby_mode.rb (run_emacs): ensure to remove a
  temporary file.

* test/misc/test_ruby_mode.rb (assert_indent): show diff between
  expected and actual results.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47202 b2dd03c8-39d4-4d8f-98ff-823fe69b080e