bug#51941: Detect duplication of ERT tests

[Mattias Engdegård]

Duplicated `ert-deftest` forms (ie, two or more definitions with the same test 
name) is a serious problem: the older test is silently ignored, which has the 
consequence of the test covering less than intended.

A quick experiment in the Emacs tree immediately reveals 51 instances. There's 
no telling how many lie waiting in external code.

Having explored various options I've come to the conclusion that it needs to be 
a hard error, at run-time, when running non-interactively (see attached diff). 
Rationale:

* We can't just warn at run time because tests tend to spam a lot when run and 
users only care about whether the tests passed and won't even look at the logs 
otherwise.
* We can't just warn at compile time because many tests aren't compiled at all, 
and where they are (as in the Emacs tree) nobody cares much about the warnings, 
because they are "just tests".
* We can't issue a complaint when running interactively because it's natural to 
keep redefining tests during development.

Since `ert-deftest` forms are often generated as a result of macro-expansion, 
passive static textual linting won't do.

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).

ert-deftest-redefine-error.diff
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