It doesn't need to expose an emulated RGB format any
longer. Skip ClearTestRGB.DefaultFramebufferRGB test
if GPUs don't support RGB format back buffer.

Bug: angleproject:4631
Change-Id: Ifa2d2ed6f863dcb698f7eea3e752a8cd5d5e0ced
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2198888Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>