The test must call glDeleteBuffers to delete a buffer instead of
glDeleteTextures. This was one reason behind the texture tests failing
on the GL backend.

BUG=angleproject:1261
TEST=angle_end2end_tests

Change-Id: I67042f6948146f3c3fca9e01231b0c4935a6f08d
Reviewed-on: https://chromium-review.googlesource.com/321611Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>