tests/test-repo-finder-mount: skip some tests if GPG is not supported
tests/test-repo-finder-mount: skip some tests if GPG is not supported

`ostree_repo_resolve_keyring_for_collection()` function fail the tests
if there is no GPG support.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

Closes: #1889
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree