run-tests: always define a custom-bin directory We are always generating `python` / `python3` executable and we will soon often generate a `hg` executable. So let's always make it a thing. This will help us to make sure we have the appropriate binary available to all script. On both linux and Windows. Differential Revision: https://phab.mercurial-scm.org/D11044

run-tests: explicitly track that `pythondir` was inferred If we want to know that something happened, we should simply track that this things happened. This will be useful to start using `tmpbindir` more in a different form. Differential Revision: https://phab.mercurial-scm.org/D11043