fix FTL_FNREDIR test for some ash derivatives
fix FTL_FNREDIR test for some ash derivatives

bin/modernish:
- Some Almquist shell derivatives (at least /bin/ash on Slackware
  and /bin/sh on NetBSD) exit when writing to a closed file
  descriptor, so stop relying on that for this test (at the cost of
  forking another command substitution subshell on every
  intialisation).

modernish/modernish