tests/cache_test: allow crossing mount points
When using an overlayfs system (like a Docker container), rmrf_local()
fails because part of the files to be removed are in different mount
points (layouts). And thus cache-test fails.
Letting crossing mount points is not a big problem, specially because
this is just for a test, not to be used in real code.
Reviewed-by:
Nicolai Hähnle <nicolai.haehnle@amd.com>