add <<TestGroup('foo')>> feature or similar (like `@groups` in PHPUnit) to run a specific subset of tests

[fredemmott]

For example, the entire test suite of docs.hhvm.com can be ran locally - but a subset, marked with @groups remote can be ran against a live deployment via HTTP.

With PHPUnit, the docs.hhvm.com deploy process was:

1. build container
2. run all tests in container
3. copy container to staging.docs.hhvm.com
4. run 'remote' tests against staging.docs.hhvm.com
5. switch cnames between staging.docs.hhvm.com with docs.hhvm.com

#4 isn't possible now

[fredemmott]

A more common case is @groups fast (expect developers to run this often) and @groups slow (expect this to pretty much only be ran by CI)

[fredemmott]

Done.