Fix compute shader in robustness tests to produce an output

Fix compute shader in robustness tests to produce an output

The compute shader only writes to shared variables, which means
that it doesn't produce any useful output. Shader compiler
backends can see this and mark all uniform variables declared
by the shader as "inactive" and remove them, which would defeat
the purpose of the 'getnuniform' test and lead to a failure.
Fix this by making the compute shader write to a shader storage
buffer object instead, so that uniforms remain active for being
involved in generating a shader output.

Component: OpenGL
VK-GL-CTS issue: 415

Affects:
KHR-GL45.robustness.getnuniform

Change-Id: I67b3fd8eb3a63012b6c46713e17c86d51b42d82e