Incorrect handling of mock ring buffer wrap-around

[fako1024]

In els0r/goProbe#88 several issues emerged when trying to implement an E2E test scenario. After some tracking down it became clear that those were encountered because the E2E test firstly created a test situation where the mock ring buffer actually ran into a wrap-around (all unit / integration tests before operated within a single "run" through the ring buffer, never exhausting its size).

This issue tracks fixes to the current implementation.