util/disk_cache: use a thread queue to write to shader cache

util/disk_cache: use a thread queue to write to shader cache

This should help reduce any overhead added by the shader cache
when programs are not found in the cache.

To avoid creating any special function just for the sake of the
tests we add a one second delay whenever we call dick_cache_put()
to give it time to finish.

V2: poll for file when waiting for thread in test
V3: fix poll delay to really be 100ms, and simplify the wait function

Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>