Insufficient integration tests about node-oauth2-server

[jankapunkt]

This is partially implemented via #224 but we will add another PR with the rest

[jankapunkt]

Whoever wants to go to implement this, we basically need in the folder /test/compliance another test implementation for authorization_code_grant_type (acgt) workflow.

Fortunately we got the other types already covered so you can take a look at the implementations in these folders and use the techniques you find in there.

Beware, that the acgt is the most complex type of all workflows and requires very thorough implementation for different scenarios (early rejections, full success, refresh etc.).

We will carefully review this one and may even add a commit here and there to support this.

[jankapunkt]

Another compliance test would be, on top of authorization_code_grant_type, to have a fully compliant pkce test setup.