Fix swarming tests not running in parallel

Due to recent Chrome infra changes in
https://chromium-review.googlesource.com/c/472290/
tests running on swarming are now assumed to emit JSON results
or will be marked as failing. This requires us to use our
gtest-parallel wrapper for all our Swarming tests
(or implement the --isolated-script-test-output flag, which
normally only is implemented by the Chromium test launcher).

The low_bandwidth_audio_test can actually run in parallel,
so just change that.

The webrtc_nonparallel_tests cannot, so this CL changes MB
to pass --workers=1 flag to gtest-parallel, which makes the
tests run in sequence. This adds a little confusion but the root
problem is really that our gtest-parallel script [1] does a lot more
than just running the tests in parallel these days, so it should
probably be renamed.

Also make sure gtest-parallel-wrapper.py [2] consumes the
--isolated-script-test-chartjson-output flag (unused) so we don't
pass it on to the test executable.

[1]: https://chromium.googlesource.com/external/github.com/google/gtest-parallel/+/master/gtest-parallel
[2]: https://chromium.googlesource.com/external/webrtc/+/master/tools-webrtc/gtest-parallel-wrapper.py

BUG=709988
TBR=ehmaldonado@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2806373002
Cr-Commit-Position: refs/heads/master@{#17646}
diff --git a/tools-webrtc/mb/mb_unittest.py b/tools-webrtc/mb/mb_unittest.py
index 6b1b2ec..998133b 100755
--- a/tools-webrtc/mb/mb_unittest.py
+++ b/tools-webrtc/mb/mb_unittest.py
@@ -441,11 +441,16 @@
 
     self.assertEqual(files, [
         '../../testing/test_env.py',
+        '../../third_party/gtest-parallel/gtest-parallel',
+        '../../tools-webrtc/gtest-parallel-wrapper.py',
         'base_unittests',
     ])
     self.assertEqual(command, [
         '../../testing/test_env.py',
+        '../../tools-webrtc/gtest-parallel-wrapper.py',
+        '--output_dir=${ISOLATED_OUTDIR}/test_logs',
         './base_unittests',
+        '--workers=1',
         '--',
         '--asan=0',
         '--msan=0',