Make use of Kyua's work directories.

Change the vnode tests to use the current directory when creating temporary
files, which we can assume is a volatile work directory, and then make the
kqueue_test.sh driver _not_ abandon the directory created by Kyua.

This makes the various kqueue tests independent of each other, and ensures
the temporary file is cleaned up on failure.

Problem spotted by asomers@ when reviewing D4254.