MFC r320974-r320975, r321001, r321206

r320974:
Use ATF cleanup routines in aio_test.c

Remove aio_test's legacy timeout handling and cleanup routines. Instead,
use ATF's builtin capabilities. ATF automatically cleans up newly created
files, too, so we don't have to explicitly unlink them. The only tests than
need a cleanup routine are the md(4) tests, which must destroy their md
device.

Reviewed by: jhb
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11468

r320975:
Add tests for aio(4) completion notification via signals and threads

Reviewed by: jhb
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11468

r321001:
Fix the build with GCC after r320975

Reported by: pfg
X-MFC-With: 320975
Sponsored by: Spectra Logic Corp

r321206:
Remove dead code that was killed by r320975

Reported by: Coverity
CID: 1377977
X-MFC-With: 320975
Sponsored by: Spectra Logic Corp