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/

src/python/pants/backend/jvm/tasks/junit_run.py

@@ -26,7 +26,6 @@
 from pants.util.dirutil import safe_mkdir, safe_open
 
 
-
 # TODO(ji): Add unit tests.
 # TODO(ji): Add coverage in ci.run (https://github.com/pantsbuild/pants/issues/83)
 
@@ -161,6 +160,9 @@ def __init__(self, task_exports, context):
       self._opts.extend(context.options.junit_run_arg)
 
   def execute(self, targets):
+    # filter out non-junit-test targets
+    targets = list(self._test_target_candidates(targets))
+
     tests = list(self._get_tests_to_run() if self._tests_to_run
                  else self._calculate_tests_from_targets(targets))
     if tests: