builtin: Reset t_wp_op in testcmd

The global variable t_wp_op needs to be reset every time testcmd
is called or it may cause incorrect parsing of the arguments.

Reported-by: Martijn Dekker <martijn@inlv.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/bltin/test.c

@@ -193,6 +193,8 @@ testcmd(int argc, char **argv)
 		argv[argc] = NULL;
 	}
+	t_wp_op = NULL;
+
 recheck:
 	argv++;
 	argc--;