glsl/tests: changes to test_disk_cache_create test

glsl/tests: changes to test_disk_cache_create test

Next patch will allow disk_cache instance to be created without
path set for it, modify some test cases that assume disk_cache
creation to fail with invalid path. Creation should succeed but
simple put/get test fail.

v2: leave tests as is but check that both cache struct exists
    and try simple put/get that should fail with invalid path set
    (Emil)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> (v1)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>