Unit testing bit-accuracy of `bit_accurate_loihi` PyProcessModels

[mathisrichter]

We should make sure, as soon as possible, that our bit-accurate PyProcessModels really have the same behavior as on Loihi. In the current absence of a Loihi implementation, we could check against the N2/N3-backend executables of the original Lava implementation (the one predating the Lava redesign). We could add these unit tests to the new Lava version already, hard-coding probed values from the Loihi implementation. Once we have an actual Lava implementation that supports Loihi, we could use these unit tests as an initial ground truth.

I am labeling this as a bug because we should really have had unit tests for this from the beginning. Also, I am seeing slightly different behavior in lava-dnf with respect to my earlier implementation based on the old Lava version. I haven't investigated that in detail but to get the same behavior on the tutorials, I had to slightly increase the synaptic strengths throughout the model.