test: add test to ensure we can use non-const lists via check.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/list/test/run-check-nonconst.c

+#include <ccan/list/list.h>
+#include <ccan/tap/tap.h>
+#include <ccan/list/list.c>
+#include "helper.h"
+
+struct child {
+	const char *name;
+	struct list_node list;
+};
+
+int main(int argc, char *argv[])
+{
+	struct child c1, c2;
+	struct list_head list = LIST_HEAD_INIT(list);
+
+	plan_tests(1);
+
+	list_add(&list, &c1.list);
+	list_add_tail(list_check(&list, "Bad list!"), &c2.list);
+	list_del_from(list_check(&list, "Bad list!"),
+		      list_check_node(&c2.list, "Bad node!"));
+	list_del_from(list_check(&list, "Bad list!"),
+		      list_check_node(&c1.list, "Bad node!"));
+	ok1(list_empty(list_check(&list, "Bad emptied list")));
+
+	return exit_status();
+}