Change parallel test logic to run only with jobs>1

Regular map is preferred when parallization does
not speed presubmit tests.  It makes stacktraces more
understandable and facilitates debugging.  Only use
parallel pool when tests > 1.

R=maruel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15385002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@200940 0039d316-1c4b-4281-b951-d872f2087c98

presubmit_support.py

@@ -478,7 +478,7 @@ class InputApi(object):
       else:
         assert issubclass(t.message, _PresubmitResult)
         tests.append(t)
-    if tests and parallel:
+    if len(tests) > 1 and parallel:
       pool = multiprocessing.Pool()
       # async recipe works around multiprocessing bug handling Ctrl-C
       msgs.extend(pool.map_async(CallCommand, tests).get(99999))