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
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11468