Fix unit tests for locally configured registries
Fix unit tests for locally configured registries

The system context is not stored any more in the image server, which
means that we cannot pass `nil` to sysregistries, because this will
access the host configured registries and might cause the unit tests to
fail.

Signed-off-by: Sascha Grunert <70ab469ddb2ac3e35f32ed7c2fd1cca514b2e879@suse.com>

kubernetes-incubator/cri-o,mikebrow/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/ocid,mikebrow/cri-o,kubernetes-incubator/ocid,nalind/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/cri-o,mikebrow/cri-o,kubernetes-incubator/cri-o,mikebrow/cri-o,nalind/cri-o