Bug 1521072 - Log wpt reftest screenshots on fail when running mach, r=ato

On CI we only want to log screenshots when something unexpected happens since anything
else is rather wasteful of resources. But locally getting screenshots for expected
failures seems helpful for debugging, so worth making the default. Hopefully this isn't
too confusing for people just checking if their patch regresses anything rather than
actively working on fixing failures.

Depends on D16973

Differential Revision: https://phabricator.services.mozilla.com/D16974

testing/web-platform/mach_commands.py

@@ -80,6 +80,9 @@ class WebPlatformTestsRunnerSetup(MozbuildObject):
         if kwargs["lsan_dir"] is None:
             kwargs["lsan_dir"] = os.path.join(self.topsrcdir, "build", "sanitizers")
 
+        if kwargs["reftest_screenshot"] is None:
+            kwargs["reftest_screenshot"] = "fail"
+
         kwargs["capture_stdio"] = True
 
         return kwargs