Add unit tests for ipdevpoll worker euthanization

Explicit coverage of the worker ping and euthanization methods did
not exist.  Instead, these methods would "accidentally" receive
coverage in some test runs, and not at all in others.  This would cause
coverage stats to flip back and forth, and codecov.io would fail
seemingly random PRs for decreasing coverage of these methods.

Fixes #2548

tests/unittests/ipdevpoll/pool_test.py

@@ -0,0 +1,30 @@
+import signal
+from unittest.mock import patch, Mock
+
+import pytest
+import pytest_twisted
+import twisted.internet.defer
+
+from nav.ipdevpoll.pool import Worker
+
+
+class TestWorker:
+    @pytest.mark.twisted
+    @pytest_twisted.inlineCallbacks
+    def test_mock_process_should_not_respond_to_ping(self):
+        worker = Worker(pool=None, threadpoolsize=0, max_jobs=5)
+        worker.process = Mock()
+        ping_response = yield worker.responds_to_ping()
+        assert not ping_response  # Mock process cannot respond
+
+    @pytest.mark.twisted
+    @pytest_twisted.inlineCallbacks
+    def test_unresponsive_worker_should_be_euthanized(self):
+        worker = Worker(pool=None, threadpoolsize=0, max_jobs=5)
+        worker._pid = 666
+        with patch.object(worker, 'responds_to_ping') as ping:
+            ping.side_effect = twisted.internet.defer.TimeoutError("Mock timeout")
+            with patch('os.kill') as mock_kill:
+                yield worker._euthanize_unresponsive_worker()
+
+                mock_kill.assert_called_with(worker.pid, signal.SIGTERM)