add some tests for annotations as dictionaries

[exaexa]

ref: #8

(We should test for the fixed state somehow, but we need to find a reasonably hosted model to be able to get it into CI.)

(cc @josePereiro )

[josePereiro]

Why not just a FBCTestModels kind of repo?

[exaexa]

Could work, but I'm not really happy to throw more resources into re-hosting other's data (which should be made properly accessible in the first place).

I think that I'll simply try to pick a random model from e.g. biomodels or bigg which features this. That requries some searching and grepping, but should be doable.