Fix mach test-wpt to make crash tests work (#29832)

* Fix `mach test-wpt` to make crash tests work

There are two issues related to crash tests:
1. test-wpt is unable to find existing crash tests even when
   called with --test-types=crashtests. The fix here is to
   add crashtests to the default test suite types to python/wpt/run.py
2. When running in headless mode, crashes in style threads
   don't cause servo to crash because the logic in constellation.rs
   currently calls handle_panic only when the top-level browsing
   context id is some value. Since style pool threads are shared,
   they always generate Panic messages with None as top-level
   browsing context id.

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

* Send bactrace to stderr and capture it in test runner

Servo's panic hook writes backtraces to stdout. This
patch changes it so they are written to stderr.

The crash test executor for servo in WPT grouping formatter
was also not capturing the output correctly for crashtests
as the log events were being aggregated based on thread name
which doesn't seem to match correctly in case of crashtests.
This patch also fixes the log grouping logic to be based on
test name.

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

* crashtests: update expectations for layout 2020

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

* crashtests: update expectations for layout 2013

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

* remove outdated & intemittent test expectations

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

---------

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

python/wpt/run.py

@@ -86,7 +86,7 @@ def run_tests(default_binary_path: str, **kwargs):
 
     if not kwargs.get("no_default_test_types"):
         test_types = {
-            "servo": ["testharness", "reftest", "wdspec"],
+            "servo": ["testharness", "reftest", "wdspec", "crashtest"],
             "servodriver": ["testharness", "reftest"],
         }
         product = kwargs.get("product") or "servo"