Migrate tests from docker.io

Switch to using images from quay.io/libpod instead, where
we're not (yet) subject to rate limiting.

Completely rewrite one unclear test. The purpose of the
test was to test #708, in which 'skopeo inspect --raw'
was fixed to be able to inspect images that do not
match the current host's os+arch. We now use a fixed
test image on quay.io, generated by a new script,
whose manifest is pretty unlikely to match our host.

Signed-off-by: Ed Santiago <santiago@redhat.com>

systemtest/030-local-registry-tls.bats

@@ -14,7 +14,7 @@ function setup() {
 @test "local registry, with cert" {
     # Push to local registry...
     run_skopeo copy --dest-cert-dir=$TESTDIR/client-auth \
-               docker://docker.io/library/busybox:latest \
+               docker://quay.io/libpod/busybox:latest \
                docker://localhost:5000/busybox:unsigned
 
     # ...and pull it back out