Tests: Remove standalone duplication. 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/+/2495281 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>