Make sure junit_run task only runs on targets that are junit compaible

Testing Done:
Two questions:

1. any suggestions on how to write a unit test for the code change? I didn't find unittest for this class.

2. how do I test the changes? Should I just create some dummy BUILD files and only put resources targets into it to make sure they're all filtered out and junit_run does an no-opt?

Thanks for bearing with me.

Bugs closed: 508

Reviewed at https://rbcommons.com/s/twitter/r/924/
Put back error message that shows path to missing BUILD files.

Regressed from  https://rbcommons.com/s/twitter/r/841/

Added a better unit test.

Testing Done:
ci passed https://travis-ci.org/pantsbuild/pants/builds/33185813

Bugs closed: 510

Reviewed at https://rbcommons.com/s/twitter/r/929/
Use target() alias in unit tests instead of dependencies()

This completely removes 'dependencies' from all the aliases in testing and
updates the unit test BUILD file mockups.