image filter test: use kubernetes/pause as a "since"
image filter test: use kubernetes/pause as a "since"

Use "kubernetes/pause", which is not expected to be updated regularly,
as a reference image for the "since" filter, instead of "alpine", which
is updated regularly, and which therefore can be newer than "busybox".

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #855
Approved by: nalind

ipbabble/buildah,nalind/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah-1,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,ipbabble/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah