tests/sign: allow to start pull test without libsodium
tests/sign: allow to start pull test without libsodium

Allow to run the pulling test if there is no ed25519 support.
Test the signed pull only with dummy engine. Fixed tests names.

Signed-off-by: Denis Pynkin <3fbf95ee68899518ee8799d35f88c8f7982aa936@collabora.com>

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