Many tests don't actually use the back buffer, so uninitialized data
is used when capturing leading to failure. Hence, clear the back buffer
when the test ends and before swap is called triggering the capture.

Bug: angleproject:6035
Change-Id: I0a500ae9f5203feaf0b492dc18e84c5a92ee1497
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2944953Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>