glsl/tests: Catch mkdir errors to help explain when they happen.

glsl/tests: Catch mkdir errors to help explain when they happen.

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.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4140>