Also cleans up some compiler warnings that crept in because of how
we structured the test sources in GN. We also no longer need special
handling for the test "main" files.

Must land after http://crrev.com/c/2495003 rolls into ANGLE.

Bug: angleproject:5124
Change-Id: I43ff91b5c8f00214886cd8ac2403702e5026a840
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2495281Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>