Refactor remove unused `test` directory
Enable RSpec support files
Add TokenGenerator public interface spec

The `TokenGenerator` clas implements the token generator duck type.
Add TokenGenerator private specs

The TokenGenerator class is responsible for the token randomness and
the fact it delegates to the Devise generator is the guaranty of its safety.

If alternative token generators appear someday, that behaviour could
become part of the class public interface.
Ensure only public interfaces specs are visible by default

The protected interfaces are those which are reasonably stable
and may be relied on in special circumstances.

The private interfaces are only tested for refactoring purpose.
Other classes must not depend on them.
Merge branch 'add-token-generator-specs'