tests/libvm.sh: Bump container run to use f31

It's the latest, and matches the rest of the host we're running on. But
also, pulling f30 is hitting 503s from the Fedora registry:

https://pagure.io/releng/issue/9282

tests/common/libvm.sh

@@ -506,7 +506,7 @@ vm_run_container() {
   # (use -n so this ssh invocation doesn't consume stdin)
   vm_cmd -n mkdir -p /var/cache/dnf
   vm_cmd podman run --rm -v /var/cache/dnf:/var/cache/dnf:z $podman_args \
-    registry.fedoraproject.org/fedora:30 "$@"
+    registry.fedoraproject.org/fedora:31 "$@"
 }
 
 # $1 - service name