remove bash-isms from /bin/sh tests

 - "not found" message is different under bash vs dash
 - "x>&y" redirects are limited to one digit.  It's actually not important
   that we find fds that aren't otherwise in use (I presume that's why all
   these were 100, 101, etc.), because "x>&y" redirection happily dups
   over in-use fds if you ask it to.
 - "exec x>&y || exit 1" redirections return exit code 2, despite the
   "exit 1" at the end.  However, "true x>&y || exit 1" redirections
   return exit code 1, as the tests expect.