Add very simple tex2lyx regression test suite.

Add very simple tex2lyx regression test suite.

It is invoked by 'make check' (automake only, it would be nice if someone
could add it to cmake as well), or by calling

python src/tex2lyx/test/runtests.py <path to tex2lyx binary>

by hand. Currently, it does not compare the output (this comes later).
The added .lyx files are from tex2lyx around mid of april, so that you
can see the regressions of the current version if you run the test
yourself (simply run git diff afterwards).

The home made test runner is quite stupid, but better than nothing.
Feel free to improve it or replace it with something better, as long
as running it stays as simple as now.