Bug 1380338 - Convert taskgraph unit tests to the |mach python-test| framework, r=dustin

These tests can now be run with:
./mach python-test taskcluster/taskgraph

or:
./mach python-test taskcluster

They can now run in parallel by passing in -j.

MozReview-Commit-ID: JXeZV8B04Sf

taskcluster/taskgraph/test/test_util_treeherder.py

@@ -6,6 +6,7 @@ from __future__ import absolute_import, print_function, unicode_literals
 
 import unittest
 from taskgraph.util.treeherder import split_symbol, join_symbol
+from mozunit import main
 
 
 class TestSymbols(unittest.TestCase):
@@ -21,3 +22,7 @@ class TestSymbols(unittest.TestCase):
 
     def test_join_with_group(self):
         self.assertEqual(join_symbol('ab', 'xy'), 'ab(xy)')
+
+
+if __name__ == '__main__':
+    main()