tests/util/grub-shell-luks-tester: Add missing line to create RET variable in cleanup

Set the RET variable to the exit status of the script, as was assumed in
the cleanup() function.

Reported-by: Thomas Schmitt <scdbackup@gmx.net>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Tested-by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

tests/util/grub-shell-luks-tester.in

@@ -132,6 +132,7 @@ fi
 
 # Make sure that the dm-crypto device is shutdown
 cleanup() {
+    RET=$?
     if [ -e "$luksdev" ]; then
         cryptsetup close "$luksdev"
     fi