tests: Shutdown contexts on completion

This should terminate any child processes and connections.

tests/id_allocation_test.py

@@ -27,3 +27,6 @@ class SlaveTest(testlib.RouterMixin, testlib.TestCase):
         # Subsequent master allocation does not collide
         c2 = self.router.local()
         self.assertEqual(1002, c2.context_id)
+
+        context.shutdown()
+        c2.shutdown()

tests/ssh_test.py

@@ -190,6 +190,7 @@ class BannerTest(testlib.DockerMixin, testlib.TestCase):
             self.dockerized_ssh.port,
         )
         self.assertEqual(name, context.name)
+        context.shutdown(wait=True)
 
 
 class StubPermissionDeniedTest(StubSshMixin, testlib.TestCase):