Add end2end tests that attempt CopyTexImage2D and CopyTexSubImage2D from
the default framebuffer.

BUG=angleproject:2027

Change-Id: Ic16527a0665317fe7d08d8204d1bd3f6309b0343
Reviewed-on: https://chromium-review.googlesource.com/494266
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>