Fix cull distance tests without fragment shader read

Fix cull distance tests without fragment shader read

These tests were supposed to test using gl_cullDistance but removing
those writes from shaders did not affect the tests passing

If fragment shader does not read cull distances the tests will now set a
cull plane and check the number of fragments culled

Components: Vulkan

VK-GL-CTS issue: 3502

Affected tests:
dEQP-VK.clipping.user_defined.clip_cull_distance.*

Change-Id: I5ea3d95cc14da934994db2b41aa22fa386570d70