test(process_registry): pin that the availability probe argv omits OOMPolicy too

Requested in the review of #102357: the spawn builder was pinned, the probe
was not, and the probe is where the rejection was cached as "unavailable".

tests/tools/test_process_registry.py

@@ -2368,6 +2368,12 @@ class TestSystemdCgroupIsolation:
         assert first is True
         assert second is True
         assert len(probe_calls) == 1, "probe must run only once (cached)"
+        # The probe must not carry OOMPolicy= either: that is the argv systemd
+        # rejected on scope units and cached as "unavailable" (#102486).
+        probe_argv = probe_calls[0][0]
+        assert not any(
+            value.startswith("OOMPolicy=") for value in probe_argv if isinstance(value, str)
+        ), probe_argv
 
     def test_systemd_scope_first_probe_is_serialized(self, monkeypatch):
         """Concurrent first-use callers must wait for one definitive probe.