tests: Fix error message for ap_open_out_of_memory

If hostapd AP started unexpectedly, this test case would fail with
NameError due to incorrect variable name being used to construct the
exception text.

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

tests/hwsim/test_ap_open.py

@@ -196,7 +196,7 @@ def hapd_out_of_mem(hapd, apdev, count, func):
         except:
             pass
         if started:
-            raise Exception("hostapd interface started even with memory allocation failure: " + arg)
+            raise Exception("hostapd interface started even with memory allocation failure: %d:%s" % (count, func))
 
 def test_ap_open_out_of_memory(dev, apdev):
     """hostapd failing to setup interface due to allocation failure"""