Tests: Add image digest test
Tests: Add image digest test

Signed-off-by: Yiqiao Pu <ypu@redhat.com>

Closes: #2004
Approved by: rhatdan

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