Add coverage for sampling compressed cubemaps that has been written

Add coverage for sampling compressed cubemaps that has been written
to by a compute shader

This commit adds new test cases to the compressed texture sampling test
to ensure that sampling compressed cubemaps works properly.

VK-GL-CTS issue: 3088

New Tests:
dEQP-VK.image.sample_texture.128_bit_compressed_format_cubemap
dEQP-VK.image.sample_texture.64_bit_compressed_format_cubemap
dEQP-VK.image.sample_texture.64_bit_compressed_format_two_samplers_cubemap
dEQP-VK.image.sample_texture.128_bit_compressed_format_two_samplers_cubemap

Components: Vulkan
Change-Id: I26724a21b03755693e45b99c8a794a9551a66e60