Call sem_unlink on semaphores before attempting to create them

Due to the lack of uniqueness in the semaphore name, and the fact that the
tests don't have cleanup routines, an interrupted test can leave a semaphore
"laying around", causing all subsequent attempts to run the test to fail

I will file a NetBSD PR for this issue soon


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274579 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f