CI: Run webdriver tests using one process and 2 chunks (#38052)

- Make the test more stable with setting one process only for each
chunk, while still running it concurrently with different machine for
each chunk.
- Previously, there is an issue where the log files is too big to be
uploaded to intermittent tracker. Now the log file should be smaller
even if we have many errors, since we have multiple chunk.

Signed-off-by: PotatoCP <Kenzie.Raditya.Tirtarahardja@huawei.com>

python/servo/try_parser.py

@@ -119,10 +119,11 @@ def handle_preset(s: str) -> Optional[JobConfig]:
                     "./tests/wpt/tests/webdriver/tests/classic/",
                     "--product servodriver",
                     "--headless",
+                    "--processes 1",
                 ]
             ),
             unit_tests=False,
-            number_of_wpt_chunks=1,
+            number_of_wpt_chunks=2,
         )
     elif any(word in s for word in ["lint", "tidy"]):
         return JobConfig("Lint", Workflow.LINT)