test-client: randomly kill or close the stub test service

The test stub service watches stdin, and if it gets closed the service
will shut down. Note that the service does not catch any signals, so
sending a signal will kill the service right away.

The previous code first closed stdin, and then killed the process.
That can result in different outcomes on D-Bus. Usually the signal
gets received first, and the test service just drops off the bus.
Sometimes it notices the closing of stdin and shuts actively down.

That can make a difference, especially for the test_monitor() test which
runs the monitor while stopping the service.

We could just always kill the stub service to get consistent behavior.
However, that doesn't seem very useful. Instead, randomize the behavior
to easier see how the behavior differs.

src/tests/client/test-client.py

@@ -656,17 +656,37 @@ class NMStubServer:
         )
         self._p = p
 
-    def shutdown(self):
+    def shutdown(self, kill_mode="random"):
         conn = self._conn
         p = self._p
         self._nmobj = None
         self._nmiface = None
         self._conn = None
         self._p = None
-        p.stdin.close()
-        p.kill()
+
+        # The test stub service watches stdin and will do a proper
+        # shutdown when it closes. That means, to send signals about
+        # going away.
+        # On the other hand, just killing it will cause the process
+        # from dropping off the bus.
+        if kill_mode == "kill":
+            p.kill()
+        elif kill_mode == "stdin-close":
+            p.stdin.close()
+        else:
+            assert kill_mode == "random"
+            ops = [p.stdin.close, p.kill]
+            random.shuffle(ops)
+            ops[0]()
+            r = random.random()
+            if r < 0.75:
+                if r < 0.5:
+                    time.sleep(r * 0.2)
+                ops[1]()
+
         if Util.popen_wait(p, 1) is None:
             raise Exception("Stub service did not exit in time")
+        p.stdin.close()
         if self._conn_get_main_object(conn) is not None:
             raise Exception(
                 "Stub service is not still here although it should shut down"