net: sockets: udp: test FIONREAD with ipv4 and ipv6 sockets

The FIONREAD ioctl (or rather ZFD_IOCTL_FIONREAD) returns the
number of bytes available on the socket that can be read
immediately.

Test that the ioctl behaves according to the expectation.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>