Add test for pet-file

We could add a test, which writes the pet-file of an empty project and then compare it to the 00_empty.pet of the demo. If there is any difference, we have changed the pet-file and should increment (at least) the Patch-version of PeTrack. Also, this test could detect problems when refactoring setXml-functions.

Note that this method will not test everything, e.g. MoCap-Persons are only written into the XML-file, when some are loaded in. Else, the section is empty.