Document how to run tests locally

As a contributor, I was unable to successfully run the tests locally. It would be great to have a few lines in the README on how to do this, without having to plough through or know much about the actual tooling to use.

Note that in the case of the more likely contributions, it would make most sense to be able to run a single pass of the tests on whatever locally installed Python, as to have a quicker evaluation - and to have the CI take care of more extensive platform-testing.