tests/unix_dgram: add test for event dispatchers

Put some data into a socket and check that:

o select(2) sees readable data
o kevent(2) sees data, and correctly sees data size
o ioctl(FIONREAD) sees correct size
o aio(4) successfully reads the data

Repeat the test twice for a connected socket and not-connected. With
future implementation these two cases would exercise different code.