Add dummy -j flags to pixel and javascript test runners

This is an intermediate step to creating a unified test
runner. We need to update Dr Memory to pass the -j 1 flag
to the pixel and javascript tests. In order to do so, we
need to have the flag available.

The flag will be enabled for pixel and javascript tests
when [1] lands.

1- https://codereview.chromium.org/1886753002/

Review URL: https://codereview.chromium.org/1882143005
diff --git a/testing/tools/run_pixel_tests.py b/testing/tools/run_pixel_tests.py
index 8a102f4..e2066e4 100755
--- a/testing/tools/run_pixel_tests.py
+++ b/testing/tools/run_pixel_tests.py
@@ -55,6 +55,9 @@
   parser = optparse.OptionParser()
   parser.add_option('--build-dir', default=os.path.join('out', 'Debug'),
                     help='relative path from the base source directory')
+  parser.add_option('-j', default=1,
+                    dest='num_workers', type='int',
+                    help='run NUM_WORKERS jobs in parallel (currently ignored)')
   parser.add_option('--wrapper', default='', dest="wrapper",
                     help='Dr. Memory wrapper for running test under Dr. Memory')
   options, args = parser.parse_args()