VK_VALVE_mutable_descriptor_type tests

VK_VALVE_mutable_descriptor_type tests

This commit adds a new group of tests to check the
VK_VALVE_mutable_descriptor_type extension. The extension allows
applications to use a new type of descriptor that doesn't have a fixed
type and can mutate among several actual descriptor types.

In addition, the extension also allows creating descriptor pools and
sets that reside entirely in host memory. They can be used as the source
for descriptor copies but cannot be bound in a command buffer.

The new tests mix mutable descriptors with nonmutable ones, arrays of
them, descriptor writes, descriptor copies, host-only and normal pool
and sets, update-after-bind, different shader stages, etc.

New tests:
dEQP-VK.binding_model.mutable_descriptor.*

Components: Framework, Vulkan
VK-GL-CTS issue: 2852

Change-Id: I19b00c2e0f56ce547a615cb39d50965f513c74db