wpt: Ensure that faulty JSON testharness output does not crash the Python test harness (#38420)

When a testharness test also prints debugging output, sometimes the
output can be mixed with the JSON output printed via an alert. This
causes a JSON decoding error in the output. Instead of crashing the
harness and printing many lines of Python stack trace output, print a
nice error. This makes the test output easier to read.

Testing: This is a change to the test harness itself, so no tests
necessary.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -521535,7 +521535,7 @@
         []
        ],
        "executorservo.py": [
-        "7b0a64a60489f91fe2d2ec92c69118c0b5b1b8c0",
+        "d983cf048ed6dfe1c07fdcac4f3465f116a00c58",
         []
        ],
        "executorservodriver.py": [

tests/wpt/tests/tools/wptrunner/wptrunner/executors/executorservo.py

@@ -173,7 +173,10 @@ class ServoTestharnessExecutor(ServoExecutor):
         prefix = "ALERT: RESULT: "
         decoded_line = line.decode("utf8", "replace")
         if decoded_line.startswith(prefix):
-            self.result_data = json.loads(decoded_line[len(prefix):])
+            try:
+                self.result_data = json.loads(decoded_line[len(prefix):])
+            except json.JSONDecodeError as error:
+                self.logger.error(f"Could not process test output JSON: {error}")
             self.result_flag.set()
         else:
             ServoExecutor.on_output(self, line)