test: Update the pinned_event integration tests code

[jmartinesp]

Changes:

• Make sure the pinned_event integration tests use MatrixMockServer behind the scenes.

• Fix a bug in the RoomEventEndpoint that caused the match_event_id option to not actually work because the event id has a $local-part:server-name format and the path_regex function of WireMock would parse the whole ^/_matrix/client/v3/rooms/.*/event/$local-part:server-name as just ^/_matrix/client/v3/rooms/.*/event/$ because it'll take the $ char as the 'end of regex' one.

• Remove unused mock_event legacy mocking method.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.43%. Comparing base (dbadfe1) to head (e913074).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4559      +/-   ##
==========================================
+ Coverage   85.40%   85.43%   +0.02%     
==========================================
  Files         285      285              
  Lines       32218    32218              
==========================================
+ Hits        27516    27524       +8     
+ Misses       4702     4694       -8     

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