Fixup test_targets_help.py and link back into :all.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/tasks/:targets_help
...
23:43:15 00:01       [jvm-compilers]
23:43:15 00:01   [test]
23:43:15 00:01     [pytest]
23:43:15 00:01       [run]
                     ============== test session starts ===============
                     platform linux3 -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2
                     plugins: cov, timeout
                     collected 2 items

                     tests/python/pants_test/tasks/test_targets_help.py ..

                     ============ 2 passed in 0.32 seconds ============

23:43:22 00:08     [junit]
23:43:22 00:08     [specs]
               SUCCESS

Bugs closed: 203

Reviewed at https://rbcommons.com/s/twitter/r/483/