tests: Fix test signer to match new API

securesystemslib main requires Signers to have a public_key property.

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

tests/test_api.py

@@ -255,6 +255,10 @@ def from_priv_key_uri(
             ) -> "Signer":
                 pass
 
+            @property
+            def public_key() -> Key:
+                raise RuntimeError("Not a real signer")
+
             def sign(self, payload: bytes) -> Signature:
                 raise RuntimeError("signing failed")