Improve the aio tests

• Add tests for aio_suspend(2).
• Add tests for polled completion notification.
• Test the full matrix of file descriptor types and completion notification mechanisms.
• Don't bother with mkstemp, because ATF runs every test in its own temp dir.
• Fix some typos.
• Remove extraneous ATF_REQUIRE_KERNEL_MODULE calls.

Reviewed by: jhb
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D9045