extend unittests

[andlaus]

Currently, test coverage of the code is quite low. This should be improved.

Andreas Lauser <andreas.lauser@mercedes-benz.com>, on behalf of MBition GmbH.
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[kayoub5]

@andlaus in order to know what's covered and what's not you will need to setup something like https://codecov.io/
I could do the codecov configuration once a it is activated on the mercedes-benz GitHub organization

[andlaus]

ok, I checked what codecov wants, but since they request read/write permissions for the entire mercedes-benz github organization (i.e., public and private repos, contributor permissions to all repos, etc.), it is not gonna fly. I've checked what coveralls (used by cantools) requests, and they only want write access to all PR statuses of the organization, which might be doable, but I need to check...