Two google sources failed to return the expected output. I looked into
each case why parsing failed:

- lyrics on musica.com contain <aside> Google Ads
- each lyrics line on lacoccinelle.net is wrapped within alternating
  <em> and <strong> tags

Thus remove these tags as part of the HTML cleanup logic.

Create 'helpers.ConfigMixin' which sets up testing configuration.
This is helpful for tests (e.g. test_lyrics.py) that only need the
configuration and do not require temp dir.

(#5102) Refactor lyrics tests to fix the issue global beets config
issue.

Additionally, add 'integration_test' mark that can be used to mark tests
that should only run once a week.

- Replaced unittest.mock with pytest fixtures for better test isolation and readability.
- Simplified test cases by using parameterized tests.
- Added `requests-mock` dependency to `pyproject.toml` and `poetry.lock`.
- Removed redundant helper functions and classes.

The test for GeniusLyrics was heavily patched and no longer provided
useful coverage. It has been removed to clean up the test suite.

- Consolidated multiple test cases into parameterized tests for better
  readability and maintainability.
- Simplified assertions by comparing lists of actual and expected
  artists/titles.
- Added `unexpected_empty_artist` marker to handle cases which
  unexpectedly return an empty artist. This seems to be happen when
  `artist_sort` field is empty.

Modified `search_pairs` function in `lyrics.py` to:

* Firstly strip each of `artist`, `artist_sort` and `title` fields
* Only generate alternatives if both `artist` and `title` are not empty
* Ensure that `artist_sort` is not empty and not equal to artist (ignoring
  case) before appending it to the artists

Extended tests to cover the changes.

Since at least one Backend requires album` and `duration` arguments
(`LRCLib`), the caller (`LyricsPlugin.fetch_item_lyrics`) must always
provide them.

Since they need to provided, we need to enforce this by defining them as
positional arguments.

Why is this important? I found that integrated `LRCLib` tests have been
passing, but they called `LRCLib.fetch` with values for `artist` and
`title` fields only, while the actual functionality *always* provides
values for `album` and `duration` fields too.

When I adjusted the test to provide values for the missing fields,
I found that it failed. This makes sense: Lib `album` and `duration`
filters are strict on LRCLib, so I was not surprised the lyrics could
not be found.

Thus I adjusted `LRCLib` backend implementation to only filter by each
of these fields when their values are truthy.

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.