test: use constant in log assertion instead of hardcoded 75

Simplify-code finding: test hardcoded exit code 75 in string
assertion while already importing GATEWAY_SERVICE_RESTART_EXIT_CODE.
Use f-string interpolation so the assertion tracks the constant.

tests/gateway/test_gateway_shutdown.py

@@ -56,7 +56,7 @@ def test_cron_provider_stop_cannot_override_gateway_exit_code(caplog):
     gateway_run._stop_cron_provider(provider)
 
     provider.stop.assert_called_once_with()
-    assert "attempted to exit the gateway with code 75; ignoring" in caplog.text
+    assert f"attempted to exit the gateway with code {GATEWAY_SERVICE_RESTART_EXIT_CODE}; ignoring" in caplog.text
 
 
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