Spin off module testing to a specific distribution

[JJ]

Testing of new distributions added to the ecosystem is not, well, tested. It's a single script, which is to a large extent Test::Meta but includes other things (and maybe should include others like duplicates #517 or, going forward, security issues. This leads to accidental discovery of problems such as #549, caused mainly to the absence of a proper test set.
So It would be convenient to create, under the Raku organization, a Test::Ecosystem module (and corresponding Docker container) that would take care of this. It would have the basic functionality the test script's got now, plus any other we could think of.