Add CopyTextureTest.ClearAfterCopySubTexture Bug: angleproject:2192 Change-Id: I75d13e089e6e47f1fd3ce62eba48e531c24abcff Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3634704 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>