Move tests logic to tox

In order to smoothly move out from the legacy QA, we should use a more convential and widely used process to manager test environments: tox.

TOXENVs

One TOXENV should be created for each kind of test (in that order of execution):

• style
• lint
• types
• cleanup
• spell
• unit
• translations (not run by default)
• ft (functional tests, not run by default)
• integration (integration tests, on Windows only, requires a Nuxeo instance, not run by default)
• bench (not run by default)

The default interpreter used should be synced with the major Python version supported, see XXX_PYTHON (search for that string in the source-code).