Test lyrics texts explicitly

Add explicit checks for lyrics texts fetched from the tested sources.

- Introduced `LyricsPage` class to represent lyrics pages for integrated
  tests.
- Configured expected lyrics for each of the URLs that are being
  fetched.
- Consolidated integrated tests in a new `TestLyricsSources` class.
- Mocked Google Search API to return the lyrics page under test.

Author: snejus