Socket.sendall expects bytes in python3, not strings
Addressing stability of pppos test: * listing active network interfaces for presence of "ppp0" interface * fixed IPv6 address regex to prevent ttfw accept partial address only (less than 8 octets) * workaround crash after exiting app_main()
Adding testing facilities for pppos-client including esp-modem component and PPP client in lwip. Testing is provided with PPP server running on rpi.