Create goldens tests

[creativecreatorormaybenot]

Description

We want to have goldens tests to generally better test the package. Additionally, we probably want to require adding goldens tests for every newly added feature.

Resources

Links to existing implementations of this feature in other TeX projects.
https://medium.com/flutter-community/flutter-golden-tests-compare-widgets-with-snapshots-27f83f266cea

[nilsreichardt]

More resources: https://github.com/lukef/qr.flutter/tree/master/test