Reduce repeated code in test_files.py.
Reduce repeated code in test_files.py.

Now we can run a set of common tests on all the dump_* functions without
repeated the code in each class.

mastizada/pontoon,jotes/pontoon,jotes/pontoon,vivekanand1101/pontoon,mathjazz/pontoon,sudheesh001/pontoon,mozilla/pontoon,mozilla/pontoon,vivekanand1101/pontoon,sudheesh001/pontoon,sudheesh001/pontoon,Jobava/mirror-pontoon,m8ttyB/pontoon,yfdyh000/pontoon,participedia/pontoon,Jobava/mirror-pontoon,yfdyh000/pontoon,yfdyh000/pontoon,mastizada/pontoon,mathjazz/pontoon,jotes/pontoon,Osmose/pontoon,mathjazz/pontoon,mastizada/pontoon,mozilla/pontoon,participedia/pontoon,mozilla/pontoon,yfdyh000/pontoon,m8ttyB/pontoon,vivekanand1101/pontoon,mathjazz/pontoon,vivekanand1101/pontoon,mastizada/pontoon,jotes/pontoon,Jobava/mirror-pontoon,m8ttyB/pontoon,Osmose/pontoon,sudheesh001/pontoon,m8ttyB/pontoon,mathjazz/pontoon,mozilla/pontoon,Osmose/pontoon,Jobava/mirror-pontoon,Osmose/pontoon,participedia/pontoon,participedia/pontoon