tests/test-gendoc-zh_TW.t

chgserver: update the umask cache before each run

posix.py uses a global variable to store the umask value resulting in caching of
it when using chg. We need to update it before each command run as the umask can
change between commands.

This fixes test-inherit-mode.t with chg.

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors