tests: fix issues with fold-characters.sh test

tests: fix issues with fold-characters.sh test

This was noticed with:

  trap '' PIPE &&  # Ignore SIGPIPE
  make check TESTS=tests/fold/fold-characters.sh SUBDIRS=.

* tests/fold/fold-characters.sh: Move memory limit test to ...
* tests/misc/write-errors.sh: ... which avoids "write error"
messages on stderr due to the ignored SIGPIPE.  It also protects
the fold invocation with a timeout(1) so that fold implementations
that don't exit promptly upon write error don't hang the test suite
(Like we would have done before commit v9.7-311-gc95c7ee76).