Bug 1568638 - [build] Create objdir in test_manifest backend if it doesn't exist r=mshal

This fixes an edge case where a task needs to invoke the TestManifest backend from a source dir that doesn't have an objdir created yet.

Depends on D43513

Differential Revision: https://phabricator.services.mozilla.com/D43514

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/gen_test_backend.py

@@ -4,6 +4,7 @@
 
 from __future__ import absolute_import, print_function
 
+import os
 import sys
 
 from mozbuild.backend.test_manifest import TestManifestBackend
@@ -22,6 +23,10 @@ def gen_test_backend():
         # to be re-created if configure runs. If the file doesn't exist,
         # mozbuild continually thinks the TestManifest backend is out of date
         # and tries to regenerate it.
+
+        if not os.path.isdir(build_obj.topobjdir):
+            os.makedirs(build_obj.topobjdir)
+
         config_status = mozpath.join(build_obj.topobjdir, 'config.status')
         open(config_status, 'w').close()