Having tox call tox is a bit weird, but the alternatives are:
- duplication of the test command, which is a bit sad
- replacing all the positive factors with conjunction of negative factors, e.g. core -> !db-!server, which makes the logic painful to follow
One day, tox will have environment aliases which will make this horror moot :-)
(Idea pulled from a comment in https://github.com/tox-dev/tox/issues/427)
Depends on D2330