Bug 1599242 - Fix edge case in test-verify reftest path substitution;…

… r=jmaher

In a minority of cases, the referenced file (here, entry[2]) has a relative
path, <subdir>/<reference-file>; since it is being joined with the full
path to the original file, the <subdir> needs to be eliminated.

Differential Revision: https://phabricator.services.mozilla.com/D54611

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

testing/mozharness/mozharness/mozilla/testing/per_test_base.py

@@ -135,7 +135,7 @@ def _find_misc_tests(self, dirs, changed_files, gpu=False):
                 # Test name substitution, for reftest reference file handling:
                 #  - if both test and reference modified, run the test file
                 #  - if only reference modified, run the test file
-                test_file = os.path.join(os.path.dirname(file), entry[2])
+                test_file = os.path.join(os.path.dirname(file), os.path.basename(entry[2]))
                 self.info("Per-test run substituting %s for %s" % (test_file, file))
                 file = test_file