bug#52971: Tests warn after eieio-compat.el was moved to obsolete/

[Stefan Kangas]

> branch: master
> commit d65534d254b3965ea82a9300c12c5c07f88818b7
> Author: Stefan Monnier <monnier@iro.umontreal.ca>
> Commit: Stefan Monnier <monnier@iro.umontreal.ca>
>
>     * lisp/emacs-list/eieio-compat.el: Really move to obsolete

After this or some other recent and related commit, we get these
warnings:

  In toplevel form:
  lisp/emacs-lisp/eieio-tests/eieio-test-methodinvoke.el:58:1:
Warning: Package eieio-compat is deprecated
    GEN      lisp/emacs-lisp/ert-x-tests.log

  In toplevel form:
  lisp/emacs-lisp/eieio-tests/eieio-tests.el:30:1: Warning: Package
eieio-compat is deprecated

I'm not sure about the best way to fix them.  Could we just move those
parts that require eieio-compat.el to a new file eieio-compat-tests.el?

I will silence these warnings using `with-no-warnings' for now, and we
can track progress on this here instead.