Refine DXT1CompressedTextureTest.DXT1Alpha to pass on more
configurations.

Fix the test's payload so that all 16 pixels use code 3.

Refine texture swizzling detection.

Bug: angleproject:2634
Bug: angleproject:5049
Change-Id: If9e1ab63220170a54954234ea205e19583b0df0a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2467896Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Le Hoang Quyen <le.hoang.q@gmail.com>
Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>