Bug 1818502 - Install `webtransport` requirements for WPT when run through mozharness r=ahal

All relevant dependencies should now be available on our internal PyPi mirror: https://pypi.pub.build.mozilla.org/pub/

See:
https://bugzilla.mozilla.org/show_bug.cgi?id=1820065
https://bugzilla.mozilla.org/show_bug.cgi?id=1820032
https://bugzilla.mozilla.org/show_bug.cgi?id=1820488

Differential Revision: https://phabricator.services.mozilla.com/D171921

testing/mozharness/scripts/web_platform_tests.py

@@ -251,6 +251,19 @@ class WebPlatformTest(TestingMixin, MercurialScript, CodeCoverageMixin, AndroidM
 
         self.register_virtualenv_module(requirements=[requirements], two_pass=True)
 
+        webtransport_requirements = os.path.join(
+            dirs["abs_test_install_dir"],
+            "web-platform",
+            "tests",
+            "tools",
+            "webtransport",
+            "requirements.txt",
+        )
+
+        self.register_virtualenv_module(
+            requirements=[webtransport_requirements], two_pass=True
+        )
+
     def _query_geckodriver(self):
         path = None
         c = self.config