Instead of having the same simple shaders repeated over and over in
the test code, reuse a single shader library.

BUG=angleproject:2474
TEST=angle_end2end_tests

Change-Id: I13f8ca8c0125e6d30f1761639bf8c3f69e0e77d2
Reviewed-on: https://chromium-review.googlesource.com/1012078Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>