Mini-SBIBM

[manuelgloeckler]

Is your feature request related to a problem? Please describe.
The SBI test suite only tests "general correctness" on rather simple tasks with analytic solutions (gaussian).

It would be very helpful to have a fast but more elaborate mini SBIBM when implementing new methods or adapting implemented ones. Simply to ensure that changes do not have an unintended impact on how well these methods work.

In contrast to the SBIBM, this should have minimal dependencies and, at best, be integrated within the pytest (probably optionally callable with flags).

Describe the solution you'd like
I think the best solution is to implement it as an additional test.
We can use pytest-harvest to collect and show results from these tests after the pytest has run.
These should likely not assert, but should highlight if performance got better/worse.

[janfb]

solved with #1335