Test vkCmdBlitImage for cubemaps with layerCount=6

Test vkCmdBlitImage for cubemaps with layerCount=6

Tests were added to verify that the typical use-case of generating
mipmap levels for cubemaps with vkCmdBlitImage() works with all supported
formats.

Tests in 'from_base_level' group use the base image as a source for all
mip levels and generate all mip levels for all six image layers with a
single blit command.

Tests in 'from_previous_level' group use the previous mip level as a
source for the next mip level and generate each mip level for all
six image layers with a separate blit command with barriers between
each blit.

Existing mipmap tests cases in the
dEQP-VK.api.copy_and_blit.*.blit_image.all_formats.generate_mipmaps.*
test group were moved to
dEQP-VK.api.copy_and_blit.*.blit_image.all_formats.generate_mipmaps.*.layercount_1.*

New tests:

dEQP-VK.api.copy_and_blit.core.blit_image.all_formats.generate_mipmaps.*.layercount_6.*
dEQP-VK.api.copy_and_blit.dedicated_allocation.blit_image.all_formats.generate_mipmaps.*.layercount_6.*

Components: Vulkan

VK-GL-CTS issue: 615
VK-GL-CTS public issue: 50

Change-Id: I512484434dda155b668a600c8303650d42417e0a