the idea behind this concept:
* have a file containing some test data
* import file without UI
* export file to svg through XStorable::storeToURL
* compare exported svg file to a reference file and complain if they
* differ

advantages:
* easy to create test cases, even for non coders
* no special handling of sd internal code needed
* svg is showing differences in the representation and not in the model
* external programs that can be used to check the output
* can be used for all input formats

disadvantages:
* changes to svg filter code might break test cases while the change is
correct
* slow test, import + export
* might need external program for xml diff