bug#51941: Detect duplication of ERT tests

[Ihor Radchenko]

Lars Ingebrigtsen <larsi@gnus.org> writes:

>> The effect of this change will be a visible and easily remedied
>> failure in broken test code. I volunteer to fix the first-order errors
>> in Emacs (by renaming the clashes); domain specialists may need to
>> help fixing secondary failures (failures in previously ignored tests).
>
> Sounds good.

This change breaks test code when a test file is "required" multiple
times. In particular, Org mode tests cannot run with latest Emacs.

Best,
Ihor