Add tests for enabling NPOT extensions.

BUG=angleproject:1678

Change-Id: Ibcbfc1192bceb634deb2904dbb9644902471e3fd
Reviewed-on: https://chromium-review.googlesource.com/425713Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>