bug#50738: `ert-deftest` bodies should not use lexical-binding=nil

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Wed, 22 Sep 2021 22:33:48 +0200
> Cc: 50738@debbugs.gnu.org
> 
> Mattias Engdegård <mattiase@acm.org> writes:
> 
> > 1. Make with-temp-buffer use the current value of lexical-binding for
> > the temporary buffer, instead of always using nil.
> 
> I think that makes the most sense?  

I think it's dangerous to do this in Emacs 28.  After all, the
original issue is with ERT, whereas the change will affect gobs of
code.  After the emacs-28 branch is cut, if you feel adventurous, ...