tests: sockets: Add tests of sendto() and recvfrom()

- Add test cases of ipv4/ipv6 sendto() and recvfrom()
- Set the main stack size to 2048 to enable both ipv4 and ipv6 on
  qemu_x86.
- Use net app for network setup.

Signed-off-by: Aska Wu <aska.wu@linaro.org>