tests: Fake datetime.now() when using mock staging

The current staging TUF assets expire in 3 days: mock datetime.now()
so they seem valid in the tests.

This is absolutely quite sketchy but seems to work.

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

Author: jku

test/unit/conftest.py

@@ -15,6 +15,7 @@
 from __future__ import annotations
 
 import base64
+import datetime
 import os
 import re
 from collections import defaultdict
@@ -158,6 +159,16 @@ def _fetch(self, url: str) -> Iterator[bytes]:
 
     monkeypatch.setattr(updater, "Urllib3Fetcher", lambda app_user_agent: MockFetcher())
 
+    # Using the staging TUF assets is a nice way to test but staging tuf assets expire in
+    # 3 days so faking now() becomes necessary. This correctly affects checks in
+    # _internal/trust.py as well
+    class mydatetime(datetime.datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return datetime.datetime(2025, 5, 6, 0, 0, 0, 0, datetime.UTC)
+
+    monkeypatch.setattr(datetime, 'datetime', mydatetime)
+
     return success, failure