Create a place and structure for extensions fixture objects

[zimeon]

It would be nice to have a place to put fixtures (both good and bad) that accompany approved extensions. It could be in this repository or in a parallel extensions-fixtures repository. Should also think about alignment with https://github.com/OCFL/fixtures

[pwinckles]

Is the intent validators should validate extensions? If so, how/where are the error codes defined. For example, if a repository is configured to use a specific storage root extension, and the validator finds an object in the repository that is not located at a path where it should not be, what error code should the validator return?