Uses a different token for skipped and not run tests in the json. The
token isn't standard but should only be visible when batches report
to the parent process that a test wasn't run. It shouldn't be visible
to the scripts that consume test results on the bots.

Also fixes the expected result for skipped tests to also be "SKIP".

Bug: angleproject:5951
Change-Id: I0bd7d13b2b25b583f9c42718786cb588bacd4bc6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2894524Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>