Enable usage of custom pylintrc file for message documentation tests (#6131)

Demonstrate it with optional checker message as example.

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

Author: DudeNr33

doc/data/messages/c/confusing-consecutive-elif/bad.py

@@ -0,0 +1,6 @@
+def myfunc(shall_continue: bool, shall_exit: bool):
+    if shall_continue:
+        if input("Are you sure?") == "y":
+            print("Moving on.")
+    elif shall_exit:  # [confusing-consecutive-elif]
+        print("Exiting.")

doc/data/messages/c/confusing-consecutive-elif/details.rst

@@ -0,0 +1 @@
+Creating a function for the nested conditional, or adding an explicit ``else`` in the indented ``if`` statement, even if it only contains a ``pass`` statement, can help clarify the code.

doc/data/messages/c/confusing-consecutive-elif/good.py

@@ -0,0 +1,22 @@
+# Option 1: add explicit 'else'
+def myfunc(shall_continue: bool, shall_exit: bool):
+    if shall_continue:
+        if input("Are you sure?") == "y":
+            print("Moving on.")
+        else:
+            pass
+    elif shall_exit:
+        print("Exiting.")
+
+
+# Option 2: extract function
+def user_confirmation():
+    if input("Are you sure?") == "y":
+        print("Moving on.")
+
+
+def myfunc2(shall_continue: bool, shall_exit: bool):
+    if shall_continue:
+        user_confirmation()
+    elif shall_exit:
+        print("Exiting.")

doc/data/messages/c/confusing-consecutive-elif/pylintrc

@@ -0,0 +1,2 @@
+[MASTER]
+load-plugins=pylint.extensions.confusing_elif

doc/test_messages_documentation.py

@@ -7,7 +7,7 @@
 from collections import Counter
 from pathlib import Path
 from typing import Counter as CounterType
-from typing import List, TextIO, Tuple
+from typing import List, Optional, TextIO, Tuple
 
 import pytest
 
@@ -53,7 +53,13 @@ def __init__(self, test_file: Tuple[str, Path]) -> None:
         self._linter.config.persistent = 0
         checkers.initialize(self._linter)
 
-        config_file = next(config.find_default_config_files(), None)
+        # Check if this message has a custom configuration file (e.g. for enabling optional checkers).
+        # If not, use the default configuration.
+        config_file: Optional[Path]
+        if (test_file[1].parent / "pylintrc").exists():
+            config_file = test_file[1].parent / "pylintrc"
+        else:
+            config_file = next(config.find_default_config_files(), None)
 
         _config_initialization(
             self._linter,