[test multiprocessing] Fix tests for unpickable plugins (#8470)

Author: Pierre-Sassoulas

tests/test_check_parallel.py

@@ -249,10 +249,9 @@ def test_linter_with_unpickleable_plugins_is_pickleable(self) -> None:
         linter.load_plugin_modules(["pylint.extensions.overlapping_exceptions"])
         try:
             dill.dumps(linter)
-            # TODO: 3.0: Fix this test by raising this assertion again
-            # raise AssertionError(
-            #     "Plugins loaded were pickle-safe! This test needs altering"
-            # )
+            raise AssertionError(
+                "Plugins loaded were pickle-safe! This test needs altering"
+            )
         except (KeyError, TypeError, PickleError, NotImplementedError):
             pass