Add more aio tests

• Add tests for kqueue completion with all file types.
• Add a test for kqueue completion with EV_ONESHOT.
• Cleanup an unused variable.

Differential Revision: https://reviews.freebsd.org/D35359

(cherry picked from commit 57082ce8c43bd555439311984fdcf738ac31dad1)

Reap dead code in lio_kqueue_test and aio_kqueue_test

Differential Revision: https://reviews.freebsd.org/D35358

(cherry picked from commit f10dd8af4b4fb17173321fac13b3221bf096555a)