tests/sys/audit: fix timeout calculation

This changes the behaviour to a 30s total timeout (needed when running
on slow emulated uniprocessor systems) and timing out after 10s without
any input. This also uses timespecsub() instead of ignoring the
nanoseconds field.

After this change the tests runs more reliably on QEMU and time out less
frequently.

Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D28391