Auto merge of #20312 - jdm:fix-manifest-update, r=asajeffrey

Avoid using WPT test runner to update the test manifest.

Our old code to update the test manifest relied on running no actual tests. This is now broken by d0a30f8821 (diff-c9390ffce43b4924882faf365b1bf136), so these changes duplicate Firefox's implementation to reduce risk of future breakage. This will allow automated WPT syncing to resume.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

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python/servo/testing_commands.py

@@ -77,6 +77,19 @@ def create_parser_wpt():
     return parser
 
 
+def create_parser_manifest_update():
+    import manifestupdate
+    return manifestupdate.create_parser()
+
+
+def run_update(topdir, check_clean=False, rebuild=False, **kwargs):
+    import manifestupdate
+    from wptrunner import wptlogging
+    logger = wptlogging.setup(kwargs, {"mach": sys.stdout})
+    wpt_dir = os.path.abspath(os.path.join(topdir, 'tests', 'wpt'))
+    manifestupdate.update(logger, wpt_dir, check_clean, rebuild)
+
+
 @CommandProvider
 class MachCommands(CommandBase):
     DEFAULT_RENDER_MODE = "cpu"
@@ -429,11 +442,9 @@ class MachCommands(CommandBase):
     @Command('update-manifest',
              description='Run test-wpt --manifest-update SKIP_TESTS to regenerate MANIFEST.json',
              category='testing',
-             parser=create_parser_wpt)
+             parser=create_parser_manifest_update)
     def update_manifest(self, **kwargs):
-        kwargs['test_list'].append(str('SKIP_TESTS'))
-        kwargs['manifest_update'] = True
-        return self.test_wpt(**kwargs)
+        return run_update(self.context.topdir, **kwargs)
 
     @Command('update-wpt',
              description='Update the web platform tests',