Was failing on Nexus 5X Adreno,
which doesn't have GL_EXT_texture_compression_dxt1 extension.
Also rename the tests to indicate they test DXT1 compression format.

BUG=angleproject:1424
TEST=DXT1CompressedTextureTestES3.PBOCompressedTexImage

Change-Id: Ic82363a6f497da12602f3f3a662a52ffa833aa0f
Reviewed-on: https://chromium-review.googlesource.com/355711Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>