Lyrics sources integration tests

Add integration tests to track which websites can be scraped by our algo and be used as sources for the google custom search engine.
Now before adding a source to the CSE, one should first add it to the *LyricsScrapingPluginTest.sourcesOk* and check that test still run ok, which confirm that the source is valid.
 
- web requests are mocked and content of html pages saved locally in `rsrc/lyrics/<sitename>/<pagename>`
- new integration tests file named with no prefix *test_* so tests are not included in the default suite