aio_md_test: fix cleanup

ATF cleanup functions cannot use functions such as ATF_REQUIRE
and atf_tc_fail. These functions assert that a test case is
currently running, which is not true during cleanup, so the
process aborts. Change the cleanup function to simply print
to stderr and return.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

(cherry picked from commit c6f92e64b6140f9e095b41687ff09ff973c28e14)