test-ndisc-fake: free l3cfg after creating fake-ndisc

Considering nm_netns_l3cfg_acquire returns a l3cfg reference and only
keeps a weak reference we need to free l3cfg in fake-ndisc after
creating the object.

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

@@ -33,6 +33,7 @@ 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);
 }