Allow pre-test and post-test hooks to be used by plugins

[pytoxbot]

• Bitbucket: https://bitbucket.org/hpk42/tox/issue/272
• Originally reported by: @Itxaka
• Originally created at: 2015-08-31T18:07:50.626

Rigth now the plugin system is kind of limited as it does not really allow to do that much.

Allowing some pre and post hooks will allow to create plugins that do actual stuff and can expand tox a lot.

real world scenario:
we run tests for an openstack dashboard on top of horizon dashboard.
we need to run a couple of docker containers to support the proper testing of our app
we want to run them locally and on jenkins
they need to be shut down automatically at the end of the tests

with tox there is no way of doing this and also DRY by sharing steps between testenvs.

By having a pre and post it would be able to create a tox-docker plugin that reads the tox config, set ups the proper containers, exposes the containers data trougth the setenvs and cleans them when the test start.

that is just an example, you coul also do the same with emails on failure, storing application logs for checking them afterwards and so on.

[gaborbernat]

https://tox.readthedocs.io/en/latest/plugins.html#tox.hookspecs.tox_runtest_post and https://tox.readthedocs.io/en/latest/plugins.html#tox.hookspecs.tox_runtest_pre