simpletest deprecation

[JamesRezo]

On github, this tool has no release since 2019, thus it must be required with dev-main when using it as aa dev dependency with composer.

The webiste, https://simpletest.org/, seems down, and https://simpletest.sourceforge.net/ is not really up-to-date :)

Can we consider replacing it with phpunit?

[Cerdic]

that would be nice, and I also had this idea. But not the time to do it, so feel welcome if you want to do it :)