Make the main py3 tox environment an alias/merge of other test environments

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)