Fix Hoa\Test with Composer

This PR does several things:
  1. `atoum/atoum` has been forked in `hywan/atoum`,
  2. a tag has been added on `hywan/atoum` to get rid of the `minimum-stability` properties in `composer.json`,
  3. we keep the `HOA_ATOUM_*` environment variables but we look for the `atoum` binary in the `vendor/bin/` directory.

The fork is temporary! See discussion and related links at https://github.com/hoaproject/Test/issues/8.
Only `hoa test:run` is up-to-date and works with Composer right now (but it is enough for the beginning!).

What is the *modus operandi* to test a library now?
  1. get a library (by cloning for example),
  2. run `composer install --dev`,
  3. run `vendor/bin/hoa test:run -d Test`.

atoum, the Praspel's extension and all dependencies are installed automatically.

I think we can open a new PR to automate the `-d Test` part if there is no file selected.

Next step:
  * update `hoa test:generate`,
  * update `hoa test:clean`,
  * update all `composer.json` of all libraries that have tests with `"require-dev": { "hoa/test": "~…" }` because `--dev` is not recursive.

When this PR will be accepted, I propose to finalize this library just after.

Thoughts?