op: Fix alpha_to_coverage_mask test for Qualcomm

[kainino0x]

It fails on Qualcomm/Adreno because it's too strict. See explanation: gpuweb/gpuweb#4867

It's hard to imagine how to test this without the test being overly strict. I've been testing various platforms (see that issue) and maybe we can just update this test such that it will pass on all platforms we were able to test, e.g.:

1. remove the extra requirement that samples not disappear as alpha increases - Adreno doesn't adhere to this
2. rather than count the samples over a 1x1 area, count the samples some multiple of a 4x4 area - 4x4 the size of the block on Adreno, which is the largest I've seen so far

[kainino0x]

Here's a collection of different behaviors to make sure the test will allow:
https://drive.google.com/drive/folders/1WFpS09pu77qie5qWqJ0aUWNM3nWAYfFW?usp=drive_link