Bug 1629781 - [wpt] Fix regression to './mach web-platform-tests' without chunking args, r=jgraham

Fixes a regression caused by bug 1614643.

Differential Revision: https://phabricator.services.mozilla.com/D70869

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

testing/web-platform/tests/tools/wptrunner/wptrunner/testloader.py

@@ -376,7 +376,7 @@ class SingleTestSource(TestSource):
 
     @classmethod
     def tests_by_group(cls, tests, **kwargs):
-        return {cls.group_metadata(None): [t.id for t in tests]}
+        return {cls.group_metadata(None)['scope']: [t.id for t in tests]}
 
 
 class PathGroupedSource(GroupedSource):