Housekeeping for create bill run validator

https://eaflood.atlassian.net/browse/WATER-4092

WATER-4092 will require us to add a new 'validator'. These are the modules that contain [Joi](https://joi.dev/) schemas we use to check inbound request payloads.

Whilst working on [Add new billing account change address validator](#383) we spotted a few things we wanted to change on the existing validator. As we only have the one, now seemed a good time to make the changes.

- fix the test filename (we'd omitted `.test`)
- move it out of the `/bill-runs` folder

On that last one, similar to [Refactor controllers out of folders](#368) if we find we have lots of validators in the future we'll likely move them into folders. But right now it feels pre-emptive.