tests/sendfile: test operation on unix/stream socket

Although there are already multiple tests in the tests collection
that utilize sendfile(2) support over unix/stream socket, they all
don't exercise the asynchronous part of the operation. This test
framework, however, uses a trick to toggle true async operation and
guarantee that pr_ready method of unix/stream is also tested.

Reviewed by: chs
Differential Revision: https://reviews.freebsd.org/D45055