tests: avoid spurious failures due to insecure directory in PATH

tests: avoid spurious failures due to insecure directory in PATH

These tests perform no PATH search, and used to simply delete PATH from
the environment.  However, that is not portable, as seen on Cygwin,
where cygwin.dll must be resolvable via PATH when starting a sub-shell.
With commit 0cc04241, we took the alternate approach of untainting the
incoming $ENV{PATH}, but that fails when it contains an other-writable
directory.  Instead, now we hard code it to '/bin:/usr/bin'.

* tests/misc/pwd-long: Hard code $ENV{PATH} to a safe value.
* tests/rm/fail-eperm: Likewise.
Reported by Gilles Espinasse, Andreas Schwab, and Bauke Jan Douma.