tests: avoid new "make distcheck" failure due to newer File::Temp

tests: avoid new "make distcheck" failure due to newer File::Temp

With newer perl (e.g., Fedora 10's 4:5.10.0-68.fc10), tests/CuTmpdir.pm
stopped removing its temporary directories, with diagnostics like this:
  cannot remove path when cwd is /c/coreutils/tests/misc/seq.tmp-e2up \
  for /c/coreutils/tests/misc/seq.tmp-e2up: at \
  /usr/lib/perl5/5.10.0/File/Temp.pm line 902
Chdir out of the target directory before that code runs:
* tests/CuTmpdir.pm (END): chdir '..'.
(chmod_tree): Remove explicit "chdir $dir".