Recent changes to truncate(2) swapped the ordering of some validations

Recent changes to truncate(2) swapped the ordering of some validations
causing EACCESS as opposed of ESDIR to be returned while trying to
truncate a directory as a user lacking write permissions to the same
directory. As this behavior is reasonable, change the truncate directory
from /etc/ to /tmp which makes the test pass both as root and non-root.