glsl/tests: fixing reliability of cache_test
A recent pipeline (https://gitlab.freedesktop.org/Venemo/mesa/-/jobs/1893357) failed with what looks like an intermittent error related to making files for the cache test inside of the core of the cache. Given some of the errors, it looks like maybe a mkdir failed, so log those errors earlier so we can debug what's going on next time.
Edited by Emma Anholt