[doc test helper] Better representation of failure for multiple bad files

[Pierre-Sassoulas]

Type of Changes

	Type
✓	🐛 Bug fix
✓	✨ New feature

Description

This starts to check that there is a warning for each files in a bad directory, except if the message is in the list of expected message that need multiple file to create an example. Result:

E           AssertionError: There should be at least one warning raised for each file in '/home/pierre/pylint/doc/data/messages/n/non-ascii-file-name/bad' (3 total)
E             - Missing warning in /home/pierre/pylint/doc/data/messages/n/non-ascii-file-name/bad/__init__.py
E             'non-ascii-file-name' might need to be added in 'known_multiple_file_messages'.
E             
E             
E           assert 2 >= 3
E            +  where 2 = len([Message(msg_id='W2402', symbol='non-ascii-file-name', msg='File name "bàd" contains a non-ASCII character.', C='W', c...ascii-file-name/bad/not_bétter.py', module='bad.not_bétter', obj='', line=1, column=0, end_line=None, end_column=None)])
E            +  and   3 = len([PosixPath('/home/pierre/pylint/doc/data/messages/n/non-ascii-file-name/bad/bàd.py'), PosixPath('/home/pierre/pylint/d...ile-name/bad/not_bétter.py'), PosixPath('/home/pierre/pylint/doc/data/messages/n/non-ascii-file-name/bad/__init__.py')])

Also fix the existing problem (no need to have an __init__.py.

Blocked by #8991

[codecov]

Codecov Report

Merging #9020 (8deecaa) into main (ee37544) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9020   +/-   ##
=======================================
  Coverage   95.75%   95.75%           
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  Files         173      173           
  Lines       18648    18648           
=======================================
  Hits        17856    17856           
  Misses        792      792           

[DanielNoord]

Suggested change

	self.multiple_file_messages = multiple_file_messages
	self._multiple_file_messages = multiple_file_messages

[Pierre-Sassoulas]

Done and rebased ;)