glsl/tests: Fix waiting for disk_cache_put() to finish.

glsl/tests: Fix waiting for disk_cache_put() to finish.

We were wasting 4s on waiting for expected-not-to-appear files to show
up on every test.  Using timeouts in test code is error-prone anyway,
as our shared runners may be busy on other jobs.

Fixes: 50989f87e62e ("util/disk_cache: use a thread queue to write to shader cache")
Link: https://gitlab.freedesktop.org/mesa/mesa/issues/2505
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4140>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4140>