test/py: net: add _lwip variants of dhcp, ping and tftpboot tests

WHen NET_LWIP is enabled, the dhcp/ping/tftpboot commands are enabled
via CMD_DHCP_LWIP, CMD_PING_LWIP and CMD_TFTPBOOT_LWIP, respectively;
therefore the config annotations in the Python test scripts need to be
cmd_dhcp_lwip, cmd_ping_lwip and cmd_tftpboot_lwip.

The console output of the tftpboot command with lwIP is slightly
different from the non-lwIP implementation. This is taken care of in
test_net_tftpboot().

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>