Under active development as of 04/2018|
The aim is to make it very easy for people to write unit or system/functional tests for fgfs (rather than simgear), which should make it much easier for new developers to dive into the flightgear sources. A long term goal might be to have good test coverage of the whole binary so that fgfs is harder to break and is more stable.
The hardest one would be to set up and run only a small subset of subsystems and other infrastructure. But it is possible to write unit tests now. Writing a system/functional test is much more difficult but, with a bit of work, it should be possible.