[test] cleanup .dot and .png files
[test] really display all the output when using --verbose
Marshalling of keys is now also well-typed
When displaying the store graph, merge the labels of edges having the same source and dest.

As in the current repr there is at most one edge between two vertex, we have to do that to not accidentaly loose string labels. Note that we do not merge other kinds of labels but we don't really care.
Fix the dump of the database for the filesystem backend.

Seems that we were not enumerating the keys properly and thus we always returned an empty store. Not sure how to unit-test that though, as the import/export values are supposed to be abstract. Let's say it's good for now on.