Bug 1515245 - Make sure windows paths are supported for wpt via generic 'mach test' r=jgraham

Differential Revision: https://phabricator.services.mozilla.com/D15497

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

testing/mozbase/moztest/moztest/resolve.py

@@ -417,6 +417,9 @@ class TestMetadata(object):
                 candidate_paths |= set(self._tests_by_path.keys())
                 continue
 
+            if os.path.sep != "/":
+                path = path.replace("/", os.path.sep)
+
             if '*' in path:
                 candidate_paths |= {p for p in self._tests_by_path
                                     if mozpath.match(p, path)}