test.only. Allow running only one test case

This is similar to TAP's `skip` feature but in reverse, it's saying skip everything else.

Use-case: pretend you have fancypants TDD tooling that runs your tests when you edit your text editor.

It's nice to be able to only run a single test case (if you have slow ass integration tests) by changing your test code instead of by context switching from your text editor to your terminal and only running a single file.