Bug 1550808 - Support reftest in sub-directory of manifest location, even on Windows; r=ahal

When TV runs the reftest harness against an individual reftest, and the specified reftest
is in a sub-directory of its manifest location, the harness currently fails to run the
requested test, on Windows only. For example, the pathname in findManifest might be
"css-multicol\multicol-nested-002.xht", on Windows. Using posixpath changes pathname to
"css-multicol/multicol-nested-002.xht", regardless of platform, and that seems to work
everywhere.

Differential Revision: https://phabricator.services.mozilla.com/D32361

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

layout/tools/reftest/runreftest.py

@@ -11,6 +11,7 @@ import json
 import multiprocessing
 import os
 import platform
+import posixpath
 import re
 import shutil
 import signal
@@ -191,7 +192,7 @@ class ReftestResolver(object):
             found = True
             while not os.path.exists(os.path.join(dirname, default_manifest)):
                 dirname, suffix = os.path.split(dirname)
-                pathname = os.path.join(suffix, pathname)
+                pathname = posixpath.join(suffix, pathname)
                 if os.path.dirname(dirname) == dirname:
                     found = False
                     break