Lesscss highlighting: Add generated reference as unit test

Whenever changing a highlighting .xml file, do the following:

1. make test
2. look at the failing tests (e.g. reference vs. generated *.html files of failing tests)
3. If failing tests need to be udpated, call in the build folder ./autotests/update-reference-data.sh
4. For new files, make sure to add the newly added files with git add.

CCMAIL: gszymaszek@short.pl