Reduce duplication in policy_config_test.go
Reduce duplication in policy_config_test.go

Most of the UnmarshalJSON tests follow the same structure,
copy&pasting boilerplate all over.  Consolidate them into a single
function with named parameters for the specifics.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image