Allow CopyTexImageTest.SubDefaultFramebuffer and
CopyTexImageTest.DefaultFramebuffer to work with Mesa and GLX back end
by doing a clear before reading the framebuffer.

BUG=angleproject:2027

Change-Id: Ibb10369e3f5aceb60511fbed1d67a28132734b29
Reviewed-on: https://chromium-review.googlesource.com/507952Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>