add test and expl for line-too-long useless-supp FP

doc/data/messages/l/line-too-long/details.rst

@@ -0,0 +1,3 @@
+If you attempt to disable this message via ``# pylint: disable=line-too-long`` in a module with no code, you may receive a message for ``useless-suppression``. This is a currently acceptable false positive for ``useless-suppression``.
+
+See https://github.com/PyCQA/pylint/issues/3368 for more information.

doc/whatsnew/fragments/3368.false_positive

@@ -0,0 +1,2 @@
+Document an acceptable false positive for ``useless-suppression`` when disabling ``line-too-long`` in a module with only comments and no code.
+Closes #3368

tests/data/line_too_long_no_code.py

@@ -0,0 +1 @@
+# ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789  # pylint: disable=line-too-long

tests/test_self.py

@@ -1247,6 +1247,21 @@ def test_encoding(self, module_name: str, expected_output: str) -> None:
             [path], expected_output=expected_output, unexpected_output="(astroid-error)"
         )
 
+    def test_line_too_long_useless_suppression(self) -> None:
+        """A test that demonstrates an acceptable false positive for useless-suppression
+
+        See https://github.com/PyCQA/pylint/issues/3368
+        """
+        module = join(HERE, "data", "line_too_long_no_code.py")
+        expected = textwrap.dedent(
+            f"""
+        {module}:1:0: I0011: Locally disabling line-too-long (C0301) (locally-disabled)
+        {module}:1:0: I0021: Useless suppression of 'line-too-long' (useless-suppression)
+        """
+        )
+
+        self._test_output([module, "--enable=all"], expected_output=expected)
+
 
 class TestCallbackOptions:
     """Test for all callback options we support."""