Bug fix for current variableSampleLocations tests + new tests

Bug fix for current variableSampleLocations tests + new tests

Support check for variableSampleLocations has been added to previously
created sample_locations_ext.verify_location tests. This CL adds also
new tests, which are now using pPostSubpassSampleLocations
correctly if variableSampleLocations == FALSE.

Components: Vulkan

VK-GL-CTS Issue: 3623

Affects:
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples_2
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples_4
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples_8
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples_16
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples*dynamic
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples*packed

New tests:
dEQP-VK.pipeline.*.sample_locations_ext.verify_location.samples*_invariable

Change-Id: I0e87c290c370ce3c534243280c40eda7b4fc6021