Add unit and simplistic functional tests for new JSON/status description
extraction functions. Because these are preview functions we also add a
stub file to add a no-op function to be built on non-preview builds.
This avoids having to reimplement large parts of the existing mirroring
tests just to validate this small aspect of the SiteMirrorImageStatus
struct.
Signed-off-by: John Mulligan <jmulligan@redhat.com>
