[9.18] fix: test: Fix check_pid() in runtime system test on FreeBSD

The original check_pid() always returned 0 on FreeBSD, even if the
process was still running. This makes the "verifying that named checks
for conflicting named processes" check fail on FreeBSD with TSAN.

Backport of MR !10373

Merge branch 'backport-mnowak/fix-runtime-pid-check-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10387