cros_run_unit_tests: run unittests for common.mk packages

Packages that use common.mk should have unittests.

BUG=chromium:867926
TEST=precq passes

Change-Id: Id894c71606247b26e43d1f5b7ee1578f697ce50f
Reviewed-on: https://chromium-review.googlesource.com/1162949
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-by: Gregory Meinke <gmeinke@chromium.org>
diff --git a/lib/portage_util.py b/lib/portage_util.py
index b37ae93..70c2acc 100644
--- a/lib/portage_util.py
+++ b/lib/portage_util.py
@@ -487,8 +487,12 @@
     is_blacklisted = False
     has_test = False
     for line in fileinput.input(ebuild_path):
-      if line.startswith('inherit ') and 'cros-workon' in line:
-        is_workon = True
+      if line.startswith('inherit '):
+        eclasses = line.split()
+        if 'cros-workon' in eclasses:
+          is_workon = True
+        if 'cros-common.mk' in eclasses:
+          has_test = True
       elif line.startswith('KEYWORDS='):
         for keyword in line.split('=', 1)[1].strip("\"'").split():
           if not keyword.startswith('~') and keyword != '-*':