lyrics: add unit tests

I wasn't sure where to put the lyrics samples (`test/lyricstext.py`), an alternative would be to put txt files in the `rsrc/` dir, but using a .py file is more straightforward/scalable for the tests.

And I thought the tests suite could be used not only to check that fetched lyrics are correct but to spot too websites that we could add to the *scraping whitelist sources* because they became "compatible" with the scraping algorithm : hence the `test_sources_incomplete` function.
I used a `print` statement to highlight such cases but it doesn't mix well with `nosetests -s` output, any idea to improve that?