ccanlint: fix -x core dump

This wasn't fixed when we converted to ccan/opt in 8d70667887.

Unfortunately, unistd.h defines optarg, so the compiler didn't catch
this.

tools/ccanlint/ccanlint.c

@@ -318,7 +318,7 @@ static char *keep_test(const char *testname, void *unused)
 
 static char *skip_test(const char *testname, void *unused)
 {
-	btree_insert(cmdline_exclude, optarg);
+	btree_insert(cmdline_exclude, testname);
 	return NULL;
 }