ci: Explicitly test shader caching
From a discussion in Chrome OS today in the context of !6967 (merged): It would be nice to end-to-end test the shader cache in CI. My thought would be to have a quick fractional gles31 run per driver after the main gles31 run, to make sure that the shader cache reloads successfully across the whole featureset of Mesa.
However, to trust that we're actually testing it, it would be nice for the second run to have a cache hit counter it could track across deqp invocations so we can verify that we did test successfully.