Fix tox environment config
The gitlab CI calls tox test by their environment name, e.g. py37-all, which are defined in tox.ini.
However, the core tests, e.g. py37-core, which are called by gitlab, are not properly defined in tox.ini. e.g. tox -l
does not lists any core environment.
This change fixed the tox.ini configuration so the core environments are recognized as properly configured by tox.