Run only related tests #13

Jest can run related tests only, either through -o for uncommitted files, or through --findRelatedTests path/to/fileA.js path/to/fileB.js to find tests related to those particular files.

If we could find a way to do this and pass the actively mutating file through we could run just the tests that we need to and massively speed up mutation testing (a few tests vs an entire suite per mutation).

Not sure if other test frameworks support this kind of thing, it'd be great to implement in a generic/extensible way.