[blinkpy] Skip unittests that use mulitple processes and write artifacts

When a unittest uses multiple processes, it's subprocesses uses the host's
file system class, not the mock file system class. This is problematic
for the blinkpy unittest step running on many builders because artifacts
from prevoius runs are not deleted on the bot.

Bug: chromium:1015118
Change-Id: I7fdf01de5ea0f782ba3ca7d4aaffda71b8841da5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866975
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707028}

third_party/blink/tools/blinkpy/web_tests/run_web_tests_unittest.py

@@ -538,6 +538,7 @@ class RunTest(unittest.TestCase, StreamTestingMixin):
             ['--order', 'natural', 'failures/unexpected/reftest-mismatch-with-text-mismatch-with-stderr.html',],
             tests_included=True, host=host))
 
+    @unittest.skip('Need to make subprocesses use mock filesystem')
     def test_crash_log_is_saved_after_delay_using_multiple_jobs(self):
         # TODO(rmhasan): When web_test_runner.run() spawns multiple jobs it uses
         # the non mock file system. We should figure out how to make all subprocesses
@@ -572,6 +573,7 @@ class RunTest(unittest.TestCase, StreamTestingMixin):
             'failures/unexpected/crash-with-sample-sample.txt',
             'retry_1/failures/unexpected/crash-with-sample-sample.txt'])
 
+    @unittest.skip('Need to make subprocesses use mock filesystem')
     def test_crash_sample_file_is_saved_multiple_jobs(self):
         host = MockHost()
         self.assertTrue(logging_run(