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> (cherry picked from commit caa616cc)