factor-parallel test failure with SHELL=zsh

[Jim Meyering]

I was surprised to see that the factor-parallel test was failing.
However, when I installed the very latest binaries early in my
path, it would succeed once again.

Turns out that when SHELL=zsh is in my environment,
the split-run "$SHELL -c factor" command was using an
PATH environment that did not have the usual .../src/: prefix,
so it would use whatever other version of factor it could
find in that PATH value.  In my case, it would use factor
from 8.23, which lacks recent patches and makes the test
fail. To show that, I ran a little experiment:

With zsh, it prints nothing for me:

  $ PATH=bogus:$PATH zsh -c 'echo $PATH'|grep bogus
  $

with bash or sh, it works as expected and prints the prefixed
value of $PATH.

This appears to be due to the fact that I set zsh's path via
my ~/.zshenv file, and that file is sourced unconditionally.
Perhaps what I am doing is best avoided? I've been doing
it for many years, so far without such a problem.
I'll send this now and investigate more later.

For me, an obvious work-around is to set SHELL=/bin/sh
or similar.