Tools: autotest: use -z in place of -v for checking variable settedness

Some bash versions out there don't like -v

Tools/autotest/run_in_terminal_window.sh

@@ -7,7 +7,7 @@ name="$1"
 shift
 echo "RiTW: Starting $name : $*"
 
-if [ ! -v SITL_RITW_MINIMIZE ]; then
+if [ -z "$SITL_RITW_MINIMIZE" ]; then
     SITL_RITW_MINIMIZE=1
 fi