Add tests for junit_run._JUnitRunner; fix a regrsssion in Emma/Cobertura; refactor exclusive_groups prep to base_test.

- Add test for junit_run._JUnitRunner class (specially testing against future regression in https://github.com/pantsbuild/pants/issues/508).
- Fix a regression introduced in last fix that impacted Emma and Cobertura (reported by jayeye)
- Refactor populate_exclusive_groups into base_test so tests (test_jvm_run and test_junit_run and others) can reuse it.

Testing Done:
- Travis CI: https://travis-ci.org/jinfeng/jinfeng-pants-fork - CI passed.

- Local run:

$ PANTS_DEV=1 ./pants goal test tests::

============== test session starts ===============
                     platform darwin -- Python 2.6.8 -- py-1.4.23 -- pytest-2.5.2
                     plugins: cov, timeout
                     collected 500 items
                     ...
                     ...
                     tests/python/pants_test/backend/jvm/tasks/test_junit_run.py .
                     ...
                     ...
                     tests/python/pants_test/tasks/test_jvm_run.py .
                     ...
                     ...

                      490 passed, 5 skipped, 5 xfailed in 413.71 seconds

16:45:57 07:37     [junit]
16:45:57 07:37     [specs]
               SUCCESS

Bugs closed: 508

Reviewed at https://rbcommons.com/s/twitter/r/935/