tests/bud.bats: add --signature-policy to some tests
tests/bud.bats: add --signature-policy to some tests

Add a --signature-policy flag to some build-using-dockerfile tests, so
that they'll pass in CI.  Check the error messages output for the cases
that we expect to fail to verify that they actually failed for the
intended reasons.

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

Closes: #1787
Approved by: rhatdan

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