Add texturegrad.sparse_samplercubeshadow back to exclude

Add texturegrad.sparse_samplercubeshadow back to exclude

It appears the following tests were removed from the exclude
list in https://gerrit.khronos.org/#/c/1819 by accident:
  dEQP-VK.glsl.texture_functions.texturegrad.sparse_samplercubeshadow_fragment
  dEQP-VK.glsl.texture_functions.texturegrad.sparse_samplercubeshadow_vertex

This CL adds them back. The non-sparse variants of these tests
are already excluded.

Affects:
dEQP-VK.glsl.texture_functions.texturegrad.*

Components: Vulkan

VK-GL-CTS issue: 877

Change-Id: Iba5e59b0a0f7c97d053084d6a52720ce020b4e21