Add tests covering variableMultisampleRate

Add tests covering variableMultisampleRate

This commit adds a new set of tests to check variableMultisampleRate
works as expected, by creating a render pass with no attachments and
multiple subpasses that use a different sample count each.

Due to the render pass and pipeline not having attachments, coverage
results are saved to a storage buffer, per sample.

Subpass results are compared to single-subpass render passes with the
same sample counts.

New tests:
dEQP-VK.pipeline.multisample.variable_rate.*

Components: Vulkan
VK-GL-CTS issue: 2152

Change-Id: I7dde8069c42f6a028754ef04a734ac980e9acf65