Active Record tests still depend on `capture`. Let's keep it for now.

This is a follow-up to 3121412

/cc @rafaelfranca

This will remove deprecation warnings from the PostgreSQL suite:

```
DEPRECATION WARNING: #capture(stream) is deprecated and will be removed in the next release. (called from capture at /Users/senny/Projects/rails/activesupport/lib/active_support/core_ext/kernel/reporting.rb:89)
/Users/senny/Projects/rails/activesupport/lib/active_support/core_ext/kernel/reporting.rb:89:in `capture'
  /Users/senny/Projects/rails/activerecord/test/cases/adapters/postgresql/composite_test.rb:73:in `ensure_warning_is_issued'
  /Users/senny/Projects/rails/activerecord/test/cases/adapters/postgresql/composite_test.rb:48:in `test_column'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:106:in `block (3 levels) in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:204:in `capture_exceptions'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:103:in `block (2 levels) in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:256:in `time_it'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:102:in `block in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:317:in `on_signal'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:276:in `with_info_handler'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:101:in `run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:759:in `run_one_method'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:293:in `run_one_method'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:287:in `block (2 levels) in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:286:in `each'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:286:in `block in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:317:in `on_signal'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:306:in `with_info_handler'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:285:in `run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:149:in `block in __run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:149:in `map'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:149:in `__run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:126:in `run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:55:in `block in autorun'
```