Add documentation of how to run tests

[c32hedge]

This might help get PR's like #124 completed. I came here after getting bitten by that bug, finding that the related issues have been open for 6 years, seeing a link to a PR from a year ago, seeing it went stale after a request to add tests, and thinking "how hard can that be?". Then I spelunked the test folder for a few minutes and...was terrified. Seems pretty ad hoc and it's not obvious where I would even start trying to run them, let alone add new ones. Maybe I'm just not familiar with the ecosystem, but I'm willing to bet I'm not the only one.