tests: keep using [] for hidden volumes

Figure out which test needs to see hidden volumes without [].
But normally use [] for debug printing in tests.
It's confusing otherwise.

test/lib/aux.sh

@@ -1074,7 +1074,6 @@ log/level = 9
 log/overwrite = 1
 log/syslog = 0
 log/verbose = 0
-report/mark_hidden_devices=0
 EOF
 	}