Bug 1853427 - Remove subsuite argument when invoked via mach test, r=Sasha

Unfortunately `mach test` makes unverified assumptions about the top
level kwargs of all test suites. This broke wpt because it was passing
around a `subsuite` argument that was previously ignored, but recently
caused a conflict in functions with signatures like
function(subsuite, **kwargs).

Fow now just remove the argument in the part of the wpt frontend that
handles data expected to be from `mach test`.

Differential Revision: https://phabricator.services.mozilla.com/D188446

testing/web-platform/mach_commands.py

@@ -502,6 +502,9 @@ def run_web_platform_tests(command_context, **params):
             params["test_types"] = list(test_types)
         params["include"] = include
         del params["test_objects"]
+        # subsuite coming from `mach test` means something more like `test type`, so remove that argument
+        if "subsuite" in params:
+            del params["subsuite"]
     if params.get("debugger", None):
         import mozdebug