Add a test for cancelling an active AIO request on a socket.

The older AIO code awakened all pending AIO requests on a socket
when any data arrived. This could result in AIO daemons blocking on
an empty socket buffer. These requests could not be cancelled
which led to a deadlock during process exit. This test reproduces
this case. The newer AIO code is able to cancel the pending AIO
request correctly.

Reviewed by: ngie (-ish)
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D4363