flow: Unit tests needed.

[ygoldfeld]

Filed by @ygoldfeld pre-open-source:

The good news is Flow has been widely tested with tons of integration testing, elsewhere. It has proven itself solid in practice.

The bad news is it lacks systematic unit testing which reduces confidence and increases risk of regressions. As an open-source project this is a deficit. Flow is manageably-sized, and it is not a hugely difficult task to resolve this. We have a pretty large GoogleTest-based unit test suite in sister/dependent project, Flow-IPC.

Since net_flow is a bit of an outlier as essentially a demo/niche module, it could be excluded from this work at first (separate ticket to handle it later/separately - TODO).