add tox.ini, orchestrating running of tests

While working on !502 (merged) and earlier contribs to this project, I had been relying on CI for running tests because it was not obvious how tests are meant to be run. Unlike Mercurial, which has the test suite built-in, evolve has some undocumented implicit steps necessary to get started with running tests (namely get a running copy of Mercurial plus a checkout of the source).

To orchestrate these steps, I put together this tox.ini file. It's completely optional to use, but provides a developer who has tox with an easy way to run the tests and manages the bootstrapping:

tox

or

tox -- --pure test-evolve.t