Don't put __snapshots__ in project root

[aaronjameslang]

No accounting for taste, but test data should be in /test or the project's equivalent

Ideally we'd detect the test directory from mocha/etc, but it could also be configurable

Would this change be welcome? Are there technical limitations an implementer should be aware of?

[kevzettler]

This should definitely be a config value or something

[hexsprite]

I tend to like the this directory to be stored in the same base directory as the test file in question. This is consistent with how Jest is configured as well I believe.