fix(deezer): use free text for singleton searches

[pszpetkowski]

Description

Singleton searches built the query as <title> artist:"<artist>". Deezer discards unquoted free text as soon as a query contains any field:"value" filter, so that was evaluated as artist:"<artist>" alone - every track by the artist, in Deezer's own relevance order, truncated to search_limit (default 5). Substituting nonsense for the title returns a byte-identical result set. For any artist with more releases than that window, the track being imported was simply never among the candidates offered.

Filtering on the title as well (track:"<title>" artist:"<artist>") is not a fix: artist: matches loosely enough to return unrelated artists, so the two filters can intersect to nothing even for a correctly tagged file. track:"Get Lucky" artist:"Daft Punk" returns zero results, while the plain free text Get Lucky Daft Punk returns the right track first.

Measured over 12 tracks at the default search_limit, counting the wanted track appearing anywhere in the results:

  <title> artist:"..."              6/12, mean rank 1.50
  track:"..." artist:"..."          7/12, mean rank 1.00
  free text                        10/12, mean rank 1.00

Album searches are unchanged; album:"<name>" has no equivalent problem.

Adds test/plugins/test_deezer.py, which did not exist.

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.63%. Comparing base (4240594) to head (681f27f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6895      +/-   ##
==========================================
+ Coverage   75.61%   75.63%   +0.02%     
==========================================
  Files         163      163              
  Lines       21323    21325       +2     
  Branches     3363     3364       +1     
==========================================
+ Hits        16124    16130       +6     
+ Misses       4406     4402       -4     
  Partials      793      793              

Files with missing lines	Coverage Δ
beetsplug/deezer.py	22.37% <100.00%> (+3.93%)	⬆️

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[semohr]

Thanks!