GN: Refactor angle_test and remove angle_test_main. This cleans up the GN files and prepares them for eventually using a custom test harness. Bug: anglerproject:3162 Change-Id: I3d8568628a9465093adec9e838617a002192beef Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1821939 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>