tests: ap_vht_csa_vht40 to check for unexpected disconnection

This test case had an error that hit an unexpected disconnection. Add an
explicit check to verify that this does not happen anymore.

Signed-off-by: Jouni Malinen <j@w1.fi>

tests/hwsim/test_ap_vht.py

@@ -743,7 +743,9 @@ def test_ap_vht_csa_vht40(dev, apdev):
             raise Exception("Channel switch event not seen")
         if "freq=5765" not in ev:
             raise Exception("Channel mismatch: " + ev)
-        time.sleep(0.5)
+        ev = dev[0].wait_event(["CTRL-EVENT-DISCONNECTED"], timeout=0.5)
+        if ev is not None:
+            raise Exception("Unexpected disconnection event from station")
         hwsim_utils.test_connectivity(dev[0], hapd)
 
         dev[1].connect("vht", key_mgmt="NONE", scan_freq="5765")