go-selinux: improve stub tests coverage
go-selinux: improve stub tests coverage

- add missing asserts
- use t.Error() instead of t.Fatal() to not fail early

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

opencontainers/selinux