Several tests need to initialize arrays of RGBA texture data. Add a
helper function for this to reduce duplication and to make the code
more readable.

BUG=angleproject:596
TEST=angle_end2end_tests

Change-Id: Id4a35b34a8fad25c2dc263ad8635dd43355a4f17
Reviewed-on: https://chromium-review.googlesource.com/333911Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>