Look at TESTMON_DATAFILE to configure datafile location

[blueyed]

I am looking into adding a "testmon" factor to pytest's tox.ini, but
noticed that this should be kept per tox environment.

Therefore the tox.ini could specify:

setenv =
    testmon: TESTMON_DATAFILE={envdir}/.testmondata

There are likely other use cases where this would come in handy.

What do you think?

TODO:

• doc

[ci skip]

[blueyed]

I have created tox-dev/tox#847 to have e.g. TOX_ENVDIR in the environment, which would make up for a good intermediate fallback (i.e. pytest's tox.ini would not have to define TESTMON_DATAFILE explicitly).

[blueyed]

@tarpas
What is your opinion on this?

I am currently looking into creating a tox plugin that would/could be shipped with testmon itself, and would adjust the default location for .testmondata then to be inside of tox's current venv.
This plugin could simply set this environment variable then.

[tarpas]

Thanks for explanation and for the code, I think it's fine.