Test suite: Fix entirely-broken tstunt PERLLIB arrangements

Specifically:
 * Properly dereference the variable when looking for the stunt
   dir to be there already
 * Do not add "." to the path if the path starts out empty.
 * Crash _with the right message_ in dpkg-parsechangelog if PERLLIB is
   unset.  (PERLLIB mustn't be unset because we're trying to strip
   our own value from it.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>