data: keep snapshot node ages when the radio clock is off (#1007)

* data: keep snapshot node ages when the radio clock is off

* data: add the snapshot clock unit tests left out of the first commit

tests/conftest.py

@@ -17,7 +17,9 @@ from __future__ import annotations
 
 import importlib
 import sys
+import time
 from pathlib import Path
+from types import SimpleNamespace
 
 import pytest
 
@@ -66,6 +68,23 @@ def permit_tx(monkeypatch):
     return config
 
 
+@pytest.fixture
+def host_clock(monkeypatch, host_now):
+    """Drive the ingestor host clock by hand, starting at ``host_now``.
+
+    ``host_now`` is a fixture the requesting test module defines, returning
+    its start time in whole Unix seconds (the receive-time checks, SPEC
+    RK1-RK4).  The clock stays there until a test moves it.
+
+    Returns:
+        A namespace whose ``now`` attribute :func:`time.time` returns.
+    """
+
+    clock = SimpleNamespace(now=host_now)
+    monkeypatch.setattr(time, "time", lambda: float(clock.now))
+    return clock
+
+
 @pytest.fixture
 def load_config(monkeypatch):
     """Reload :mod:`~data.mesh_ingestor.config` under the given environment.