Fix tex2lyx file format comparison

Fix tex2lyx file format comparison

This is an important part of the tests. If updating the test cases is really a
problem there are two better solutions than not testing the format: Convert
the references with lyx2lyx on the fly, or remove the hard coupling of tex2lyx
and LyX versions again. Both increase additional possible error sources, but
these errors could at least be detected. If the test machinery is made blind
for versions, file format errors are impossible to detect.