init.sh: do not rely on autoupated PWD

init.sh: do not rely on autoupated PWD

This addresses symptoms of the problem reported by Nelson H.F. Beebe in
<http://lists.gnu.org/archive/html/bug-gzip/2012-06/msg00008.html>.
Although Nelson's bug was not necessarily fixed by this patch,
it seems wise to make the change for safety.
* tests/init.sh (path_prepend_): Do not rely on PWD updating
automagically after 'cd'; this is not reliable on older shells.
(setup_): Fail if we cannot cd to temporary directory.