shell: add OPTARG poisoning to getopt_optarg.tests

ash fails this!

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash_test/ash-getopts/getopt_optarg.tests b/shell/ash_test/ash-getopts/getopt_optarg.tests
index b346284..33682e8 100755
--- a/shell/ash_test/ash-getopts/getopt_optarg.tests
+++ b/shell/ash_test/ash-getopts/getopt_optarg.tests
@@ -4,6 +4,7 @@
 OPTARG=ASDFGH
 while getopts "w:et" var; do
 	echo "var:'$var' OPTIND:$OPTIND OPTARG:'$OPTARG'"
+	OPTARG=ASDFGH
 done
 echo "exited: var:'$var' OPTIND:$OPTIND OPTARG:'$OPTARG'"
 
@@ -12,5 +13,6 @@
 OPTIND=0
 while getopts "w:et" var; do
 	echo "var:'$var' OPTIND:$OPTIND OPTARG:'$OPTARG'"
+	OPTARG=ASDFGH
 done
 echo "exited: var:'$var' OPTIND:$OPTIND OPTARG:'$OPTARG'"