tests: add nucleo32-f031 to unsufficient memory boards

tests/conn_ip/Makefile

tests/driver_pir/Makefile

tests/driver_xbee/Makefile

tests/gnrc_ipv6_ext/Makefile

tests/gnrc_sixlowpan/Makefile

tests/gnrc_sock_ip/Makefile

tests/gnrc_sock_udp/Makefile

tests/gnrc_tcp_client/Makefile

tests/gnrc_tcp_server/Makefile

tests/irq/Makefile

tests/libfixmath_unittests/Makefile

tests/lwip/Makefile

tests/lwip_sock_ip/Makefile

tests/lwip_sock_tcp/Makefile

tests/lwip_sock_udp/Makefile

tests/msg_send_receive/Makefile

tests/msg_try_receive/Makefile

tests/mutex_order/Makefile

tests/mutex_unlock_and_sleep/Makefile

tests/netdev2_test/Makefile