tests: fix false failure under some shells

tests: fix false failure under some shells

Verified with:
  make -j4 PREFERABLY_POSIX_SHELL=/bin/ksh SUBDIRS=. check

* tests/fold/fold-zero-width.sh: Don't timeout $SHELL -c ...
as the ulimit induces a failure in the subshell depending
on the order of the allocations it does.  The main issue is
disparity between the probed ulimit and that needed by $SHELL -c.
Such subshells load the often very large locale archive, thus
if there are any allocations done after the now too low ulimit is set,
then the $SHELL command fails. Note we timeout fold rather than
the whole pipeline so any 124 exit status is propagated.