Hi,

While testing the update of Zsh 5.6 for GNU Guix [0] I encountered some
new test failures in the Zsh test suite.

Specifically, like this:

------
Test ./A05execution.ztst failed: bad status 127, expected 0 from:
  PATH=/bin:${ZTST_testdir}/command.tmp/ tstcmd-slashless
Error output:
(eval):1: /tmp/guix-build-zsh-5.6.drv-0/zsh-5.6/Test/command.tmp//tstcmd-slashless: bad interpreter: sh: no such file or directory
Was testing: path (3)
./A05execution.ztst: test failed.
------

Indeed, in the build environment of Guix packages, there is no /bin/sh
or any other executables at locations that might be familiar and
possible to guess in advance. There is a `sh` (from Bash) on PATH, but
these tests reset PATH.

The next test in the file failed similarly, but because it is missing
`echo` rather than `sh`.

Our workaround can be seen here:

https://git.savannah.gnu.org/cgit/guix.git/commit/?id=fcf4841c0ee16631e8330d797cf3788dac0eca05

[0] https://www.gnu.org/software/guix/