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>

tools/wptrunner/wptrunner/executors/executorservo.py

@@ -83,6 +83,7 @@ def teardown(self):
         ProcessTestExecutor.teardown(self)
 
     def do_test(self, test):
+        self.test = test
         self.result_data = None
         self.result_flag = threading.Event()
 
@@ -156,7 +157,7 @@ def on_output(self, line):
             else:
                 self.logger.process_output(self.proc.pid,
                                            line,
-                                           " ".join(self.command))
+                                           " ".join(self.command), self.test.url)
 
     def on_finish(self):
         self.result_flag.set()
@@ -270,18 +271,19 @@ def screenshot(self, test, viewport_size, dpi, page_ranges):
                 return True, [base64.b64encode(data).decode()]
 
     def do_test(self, test):
+        self.test = test
         result = self.implementation.run_test(test)
 
         return self.convert_result(test, result)
 
-    def on_output(self, line):
+    def on_output(line):
         line = line.decode("utf8", "replace")
         if self.interactive:
             print(line)
         else:
             self.logger.process_output(self.proc.pid,
                                        line,
-                                       " ".join(self.command))
+                                   " ".join(self.command), self.test.url)
 
 
 class ServoTimedRunner(TimedRunner):
@@ -342,7 +344,7 @@ def do_crashtest(self, protocol, url, timeout):
         env["HOST_FILE"] = self.hosts_path
         env["RUST_BACKTRACE"] = "1"
 
-        command = build_servo_command(self.test,
+        self.command = build_servo_command(self.test,
                                       self.test_url,
                                       self.browser,
                                       self.binary,
@@ -351,16 +353,23 @@ def do_crashtest(self, protocol, url, timeout):
                                       extra_args=["-x"])
 
         if not self.interactive:
-            self.proc = ProcessHandler(command,
+            self.proc = ProcessHandler(self.command,
                                        env=env,
+                                       processOutputLine=[self.on_output],
                                        storeOutput=False)
             self.proc.run()
         else:
-            self.proc = subprocess.Popen(command, env=env)
+            self.proc = subprocess.Popen(self.command, env=env)
 
         self.proc.wait()
 
         if self.proc.poll() >= 0:
             return {"status": "PASS", "message": None}
 
         return {"status": "CRASH", "message": None}
+
+    def on_output(self, line):
+        line = line.decode("utf8", "replace")
+        self.logger.process_output(self.proc.pid,
+                                   line,
+                                   " ".join(self.command), self.test.url)