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/expected.py

@@ -0,0 +1,18 @@
+# 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/.
+
+import os
+
+
+def expected_path(metadata_path, test_path):
+    """Path to the expectation data file for a given test path.
+
+    This is defined as metadata_path + relative_test_path + .ini
+
+    :param metadata_path: Path to the root of the metadata directory
+    :param test_path: Relative path to the test file from the test root
+    """
+    args = list(test_path.split("/"))
+    args[-1] += ".ini"
+    return os.path.join(metadata_path, *args)