Add docs for AS::TestCase::test_order

Document `test_order` and `test_order=` from `ActiveSupport::TestCase`.

[ci skip]
Merge pull request #18077 from claudiob/add-docs-for-test-order

Add docs for AS::TestCase::test_order
Enable Travis CI container-mode

And also bundler cache
Output extension is no longer generated [ci skip]

/cc @senny Is this change documented somewhere?