I once briefly experimented with the idea of a programmable fake server:

1. At startup, the fake server is tabula rasa

2. The "before" part of the API test script will call specialised POST endpoints of the server to set up the fake endpoints (and the responses they are supposed to send) required for each test. The state is in-memory.

3. When the service that is the test target is started in test mode, all its dependencies point to the fake server.

4. The test script then runs the test on the service

This required zero application level mocking code. All the mocks were defined in the test scripts.