Add special tests for environment or feature cases

[sveetch]

For some specific cases depending from packages or low level configuration, it is too difficult to test it correctly in the base test suite.

It would be helpful to make other specific test suites to run explicitely (with a new make "test-something" action) and its own test settings file.

This is not currently required but writed here for possible further feature.

[sveetch]

This could be done starting with Tox environments config and specific settings.

[sveetch]

Currently the tox configuration is enough, in future with a feature like #7 it will be a matter of concern, but for now there is no need anymore.