Lower the barrier to contribute IEF tests

Problem/Motivation

Implementing new features in IEF is always prone to regressions, as IEF code has grown so convoluted over the years.
A lot of code is in the pipeline, but tests are almost always the limiting factors.
Even for a good developer, it is not clear, how to add to the tests.

Proposed resolution

- Add documentation and pointers for people that want to add tests.