Allow further customization of where snapshots are stored

Per discussion, in an ava.config.js:

export default {
  snapshotDir (testFile) {
    return new URL('.', testFile)
  }
}

Up till now snapshotDir is a fixed directory under which snapshot files are stored, with subdirectories that match the relative location of the corresponding test files. My suggestion is we make it a function which receives a file URL string (like import.meta.url) and returns a file URL instance that points at the directory within which the snapshots should be stored. The above example would store them alongside the test files.

The main process will have to run this function before it spins up the test worker thread and pass the resulting directory to the runner.