bug#47138: 28.0.50; comp-tests suite creates a /nonexistent directory

[Eli Zaretskii]

> Cc: 47138@debbugs.gnu.org
> Date: Sun, 21 Mar 2021 17:30:36 +0000
> From:  Andrea Corallo via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> if we have a better idea on how to detect we are running the testsuite
> we can use that in place.  Maybe defining a dedicated env variable
> during the build?

Howe about EMACS_ELN_PATH, similar to EMACS_LOADPATH?  Then the test
Makefile could set it for the src/comp-tests test.