ndisc/tests: fix reference counting in nm_fake_ndisc_new()

This adjusts the change from commit ffbcf01589 ('test-ndisc-fake:
free l3cfg after creating fake-ndisc').

ndisc_new() already correctly handles the reference count of l3cfg via
"gs_unref_object". The party that took the wrong reference was
nm_fake_ndisc_new().

Fixes: 58287cbcc0 ('core: rework IP configuration in NetworkManager using layer 3 configuration')

src/core/ndisc/nm-fake-ndisc.c

@@ -358,7 +358,7 @@ NMNDisc *
 nm_fake_ndisc_new(NML3Cfg *l3cfg)
 {
     const NMNDiscConfig config = {
-        .l3cfg                        = g_object_ref(NM_L3CFG(l3cfg)),
+        .l3cfg                        = NM_L3CFG(l3cfg),
         .ifname                       = nm_l3cfg_get_ifname(l3cfg, TRUE),
         .node_type                    = NM_NDISC_NODE_TYPE_HOST,
         .stable_type                  = NM_UTILS_STABLE_TYPE_UUID,

src/core/ndisc/tests/test-ndisc-fake.c

@@ -33,7 +33,6 @@ ndisc_new(void)
     memset(&iid, 0, sizeof(iid));
     iid.id_u8[7] = 1;
     nm_ndisc_set_iid(ndisc, iid, FALSE);
-    g_object_unref(l3cfg);
 
     return NM_FAKE_NDISC(ndisc);
 }