Provide a pytest fixture to mock auth

[Colin-b]

For now it's a bit painful for users as they need to know what to mock and how.

Fixture should send a token containing the information as provided by the user (with default values for users that may not know the content of a token).

Fixture should also allow to test for various authentication failures sent by requests_auth so that user can test that code handles it properly.

[Colin-b]

Available in develop branch. If there is a need for a release please say so.