Add support for running a command before tests.

This depends on https://rbcommons.com/s/twitter/r/1177/

Testing Done:
Ran new test.

Reviewed at https://rbcommons.com/s/twitter/r/1179/
Sync of https://github.com/pantsbuild/pants/commit/633bd56b4799eb6c3ea3a37571ffbcb39c973cd8