Add document that you rake test single file by using DB(ex. postgresql)

[yukihirop]

• I want to rake test by using DB(ex. postgresql) for single file.
• When I see activerecord/RUNNING_UNIT_TESTS.rdoc, I can not find.
• I add document.

I try like that:

$ bundle exec rake test:postgresql TEST=test/cases/base_test.rb
/Users/fukudayukihiro/.rbenv/versions/2.4.1/bin/ruby -w -I"lib:test" -I"/Users/fukudayukihiro/RubymineProjects/rails/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib" "/Users/fukudayukihiro/RubymineProjects/rails/vendor/bundle/ruby/2.4.0/gems/rake-12.0.0/lib/rake/rake_test_loader.rb" "test/cases/base_test.rb" 
Using postgresql
Run options: --seed 4420

# Running:

...................................................................................................................................................

Finished in 2.544319s, 57.7758 runs/s, 135.9892 assertions/s.
147 runs, 346 assertions, 0 failures, 0 errors, 0 skips

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @kaspth (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

This repository is being automatically checked for code quality issues using Code Climate. You can see results for this analysis in the PR status below. Newly introduced issues should be fixed before a Pull Request is considered ready to review.

Please see the contribution instructions for more information.

[kaspth]

Sure, thanks!