Refactor the tests suite to speed up execution

We have to refactor tests to take way less time to complete.
Currently:

• GNU/Linux: 1h30
• Windows: 1h42
• macOS: 1h55

For every little change, we have to wait at least 2h, so a refactoring can only be helpful.

Ideas:

• rework setUp and setUpServer
• parallelize tests (each test file will work into its own Nuxeo workspace using the class name as directory name per example)

Steps:
1. use only one base class
2. use addCleanup for all kind of cleanup (should fix NXDRIVE-317)
3. fix bad logging use introduced with NXDRIVE-335
4. permit several Drive instances
5. try xdist plugin to parallelize all tests
6. eventually rethink wait_async