Allow sample counting after multisample coverage

Allow sample counting after multisample coverage

Vulkan specification was modified to allow counting samples
after mutlisample coverage and fragment shading.

The test uses a fragment shader that kills all pixels with
"gl_SampleMask[0] = 0x0" and than verifies the occlusion query results.

This change allows for 0 samples to be counted by a query in case
of `early_fragment_tests` mode being used. The QualityWarning is emitted
in this case.

Components: Vulkan

Vulkan Issue: 3085
VK-GL-CTS Issue: 3778

Affects: dEQP-VK.fragment_operations.early_fragment.sample_*

Change-Id: I86cf17b2d8849a9d543e8447569ac808928c30e6