taggers: french/english melt variants transformed from custom constructor into custom subclasses (in order for them to be compatible with constants.load_tagger)

unittests: use TestCase.setUpClass for the test node creation (this way it will only be created once)

unittests: fixed framework to be able to use gargantext.util.db.session seemlessly on the test db as if it was the normal one (no need anymore for GargTestRunner.testdb_session)

unittests: factorize (same tests for different resources) + start adding array of expected ndocs per test samples