test/integration: ability to skip store perm check

Add the ability to skip the store failure tests. This is necessary because they rely on DAC (discretionary access control) of the file in the store, i.e. the test removes the permission to write to the file. If the test executor has CAP_DAC_OVERRIDE, it can bypass that and thus the test wont work. Consolidate the two store failure checks (label, policy) into an extra test and skip that if the caller has CAP_DAC_OVERRIDE. NB: "ninja -C build test" will still show the test as Ok (and the log will tell it was skipped) because all the tests are executed individually and the result then will be Ok(skipped=1) which is not picked up by meson.