Bug 1122590 - Make |mach mochitest| properly detect devtools tests (and other subsuites), r=chmanchester, DONTBUILD, a=NPOTB

--HG--
extra : commitid : F1JLpseHkrE
extra : rebase_source : 95a5f0516e286a1f9752c39ebc23ebb6349b33c6
extra : amend_source : 90c4fe59dc89dfce41d616326ce8e20e44c04e9a

testing/mochitest/mach_commands.py

@@ -553,6 +553,7 @@ class MachCommands(MachCommandBase):
             print(NOW_RUNNING.format(msg))
 
             harness_args = kwargs.copy()
+            harness_args['subsuite'] = subsuite
             harness_args.update(fobj.get('extra_args', {}))
 
             result = run_mochitest(