Add downstream test discrepency reporter

Example Run and Output:

```
$ python tests/report_downstream_test_diffs.py --playwright-js-path=../playwright --api=async

MISSING, MISPELLED, OR MISNAMED:
async/test_accessibility.py::test_autocomplete
…
async/test_workers.py::test_should_have_JSHandles_for_console_logs
async/test_workers.py::test_should_report_console_logs
async/test_workers.py::test_should_report_errors
async/test_workers.py::test_should_report_network_activity
async/test_workers.py::test_should_report_network_activity_on_worker_creation

Missing: 1213, Found: 105
```

Relates #138.

Author: rwoll

tests/report_downstream_test_diffs.py

@@ -0,0 +1,85 @@
+import argparse
+import json
+import os
+import re
+import subprocess
+import tempfile
+from collections import namedtuple
+
+TestCase = namedtuple("TestCase", ["api", "file", "test"])
+
+
+def pytest_test_cases():
+    p = subprocess.run(
+        ["pytest", "--browser", "chromium", "--collect-only", "-q"],
+        stdout=subprocess.PIPE,
+    )
+    regex = (
+        r"tests/(?P<api>a?sync)/test_(?P<file>.*)\.py::test_(?P<test>.*)\[chromium\]"
+    )
+    matches = re.finditer(regex, p.stdout.decode(), re.MULTILINE)
+    for match in matches:
+        yield TestCase(
+            match.group("api"), match.group("file"), match.group("test"),
+        )
+
+
+def jest_test_cases(playwright_js_path):
+    env = os.environ.copy()
+    env["REPORT_ONLY"] = "true"
+    with tempfile.NamedTemporaryFile() as results:
+        subprocess.run(
+            ["npx", "jest", "--json", "--outputFile", results.name],
+            env=env,
+            cwd=playwright_js_path,
+            stderr=subprocess.DEVNULL,
+        )
+
+        for test_suite in json.load(results)["testResults"]:
+            regex = r"(.*/)?(?P<file>[^/]+)\.spec\.[jt]s$"
+            file = re.match(regex, test_suite["name"]).group("file")
+            for assertion in test_suite["assertionResults"]:
+                yield TestCase("sync", normalized(file), normalized(assertion["title"]))
+                yield TestCase(
+                    "async", normalized(file), normalized(assertion["title"])
+                )
+
+
+def normalized(original):
+    return re.sub(r"[^a-z_]", "_", original, flags=re.IGNORECASE)
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "--playwright-js-path",
+        type=str,
+        help="path to playwright JavaScript directory",
+        required=True,
+    )
+    parser.add_argument(
+        "--api",
+        type=str,
+        help="filter test cases based on API",
+        choices=["sync", "async"],
+    )
+    args = parser.parse_args()
+
+    python_tests = set(pytest_test_cases())
+    javascript_tests = set(jest_test_cases(args.playwright_js_path))
+
+    if args.api:
+        javascript_tests = set([x for x in javascript_tests if x[0] == args.api])
+
+    missing = javascript_tests.difference(python_tests)
+    found = javascript_tests.intersection(python_tests)
+
+    print("MISSING, MISPELLED, OR MISNAMED:")
+    for (api, file, test) in sorted(missing):
+        print(f"{api}/test_{file}.py::test_{test}")
+
+    print(f"\nMissing: {len(missing)}, Found: {len(found)}")
+
+
+if __name__ == "__main__":
+    main()