Insufficient integration tests

[jankapunkt]

Specify your setup

• version of @node-oauth/oauth2-server: 5.0.0-rc.1

Describe the bug

The integration tests do not properly cover the regressions, introduced by the switch to native async/await. This is due to the original integration tests do not really test, whether the correct values are passed to the model as most model functions are not covering parameters or return nearly nothing.

This already resulted in:

• Client Credentials broken in 5.0.0-rc.1 #217
• generateAuthorizationCode not being awaited #204

Proposal:

All integration tests should also cover proper model testing.

We should target this for 5.0.0, because there may be more parts affected as may have been revealed yet.

[jankapunkt]

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