tests/libtest: add functions for ed25519 tests
tests/libtest: add functions for ed25519 tests

Add functions for keys generation to be used in signing-related tests:
- gen_ed25519_keys initializing variables ED25519PUBLIC, ED25519SEED and
  ED25519SECRET with appropriate base64-encoded keys
- gen_ed25519_random_public print a random base64 public key (used in
  tests with wrong keys)

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

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