Create detailed test suite (checking the results)

[oscargus]

It would be good to try to implement some tests where the VCD file is read and the results of parsing is checked.

Probably a hand-crafted file to get as many features as possible in (and just as many events as required).

The current approach of reading lots of generated files makes sense in many ways (and should be kept), but it is also good to be able to confirm less common features and that the parsing is correct/consistent (to avoid e.g. the discussion in #19, while not a big deal it still took some time for all of us).