Put a copy of the wptrunner harness in-tree.

This is the same configuration as gecko and is convenient for making changes compared
to using releases from pypi

tests/wpt/harness/wptrunner/executors/process.py

@@ -0,0 +1,23 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from .base import TestExecutor
+
+
+class ProcessTestExecutor(TestExecutor):
+    def __init__(self, *args, **kwargs):
+        TestExecutor.__init__(self, *args, **kwargs)
+        self.binary = self.browser.binary
+        self.interactive = self.browser.interactive
+
+    def setup(self, runner):
+        self.runner = runner
+        self.runner.send_message("init_succeeded")
+        return True
+
+    def is_alive(self):
+        return True
+
+    def do_test(self, test):
+        raise NotImplementedError