Add more test related to early fragment tests

Add more test related to early fragment tests

- Added tests for EarlyFragmentTest followed by a shader which
  discards. Then check that depth buffer is updated but the color
  buffer and other outputs are not.

- Added tests that write to gl_SampleMask and checked that do not affect
  the depth values written to the depth buffer.

Components: Vulkan
VK-GL-CTS issue: 2968

Added tests:

   dEQP-VK.fragment_operations.early_fragment.discard*
   dEQP-VK.fragment_operations.early_fragment.samplemask*

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Change-Id: I4a0bd49ab9319747e8c92aaa26f64a4e077fa8a5