The `test_ipv6_group_join__ENOMEM()` tests joining multicast groups, so we need to iterate `GNRC_NETIF_IPV6_GROUPS_NUMOF` times to fill up all addresses, not `GNRC_NETIF_IPV6_ADDRS_NUMOF`.
The `test_ipv6_group_join__ENOMEM()` tests joining multicast groups, so we need to iterate `GNRC_NETIF_IPV6_GROUPS_NUMOF` times to fill up all addresses, not `GNRC_NETIF_IPV6_ADDRS_NUMOF`.