Fix validation errors in ASTC decode mode tests

Fix validation errors in ASTC decode mode tests

The tests were trying to compare the raw contents of each ASTC
block contents between the tested and reference images via UINT and
SINT image views, but the type of the image view must be compatible
with the image format.

This CL changes the test to compare individual sampled texel
values instead of raw ASTC texel block values. The generated
images now contain valid ASTC blocks instead of random values.

Affects:
dEQP-VK.image.astc_decode_mode.*

Components: Vulkan

VK-GL-CTS issue: 2820

Change-Id: I38113725692f754c5de4393b5fad5d333cf99533