Lyrics integration sources

**Supersedes PR #715**

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