make test-skopeo: only run unit tests
make test-skopeo: only run unit tests

Only run Skopeo's unit tests and skip the validation process; running
linters on Skopeo's code is not up to containers/image.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

containers/image,containers/image