test: add missing ASSERT_OK()

Follow-up for 3f4a43f0eb.

Addresses https://github.com/systemd/systemd/pull/43733#discussion_r4087916957

(cherry picked from commit 63850321af)

src/libsystemd-network/test-ndisc-option.c

@@ -290,7 +290,7 @@ TEST(ndisc_dnssl) {
 
 TEST(ndisc_captive_portal) {
         _cleanup_set_free_ Set *options = NULL;
-        ndisc_option_set_captive_portal(&options, "https://captive-portal.example.com");
+        ASSERT_OK(ndisc_option_set_captive_portal(&options, "https://captive-portal.example.com"));
         ndisc_round_trip(options);
 }