add missing tests file

tests/test_shutdown.py

@@ -0,0 +1,22 @@
+from unittest.mock import MagicMock
+
+from kube_janitor.shutdown import GracefulShutdown
+
+
+def test_graceful_shutdown(monkeypatch):
+    handler = GracefulShutdown()
+    assert not handler.safe_to_exit
+    with handler.safe_exit():
+        assert handler.safe_to_exit
+
+    assert not handler.shutdown_now
+    # this would be called by SIGINT or SIGTERM
+    handler.exit_gracefully(None, None)
+    assert handler.shutdown_now
+
+    mock_exit = MagicMock()
+    monkeypatch.setattr('sys.exit', mock_exit)
+    with handler.safe_exit():
+        handler.exit_gracefully(None, None)
+
+    mock_exit.assert_called_once_with(0)