Emulate CLI to discard false symlink detection

The test attemps to emulate the behavior of the CLI as closely as
posible by patching some `pathlib.Path` methods and passing certain
reference paths to the context object and `black.get_sources`.

Before the associated fix was introduced, this test failed because
some of the captured files reported the presence of a symlink due to
an incorrectly formated path. The test also asserts that only a single
file is reported as ignored, which is part of the expected behavior.

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

tests/test_black.py

@@ -475,6 +475,53 @@ def test_tab_comment_indentation(self) -> None:
         self.assertFormatEqual(contents_spc, fs(contents_spc))
         self.assertFormatEqual(contents_spc, fs(contents_tab))
 
+    def test_false_positive_symlink_output_issue_3384(self) -> None:
+        # Emulate the behavior when using the CLI (`black ./child  --verbose`), which
+        # involves patching some `pathlib.Path` methods. In particular, `is_dir` is
+        # patched only on its first call: when checking if "./child" is a directory it
+        # should return True. The "./child" folder exists relative to the cwd when
+        # running from CLI, but fails when running the tests because cwd is different
+        project_root = Path(THIS_DIR / "data" / "nested_gitignore_tests")
+        working_directory = project_root / "root"
+        target_abspath = working_directory / "child"
+        target_contents = (
+            src.relative_to(working_directory) for src in target_abspath.iterdir()
+        )
+
+        def mock_n_calls(responses: List) -> Callable[[], bool]:
+            def _mocked_calls() -> bool:
+                if responses:
+                    return responses.pop(0)
+                return False
+
+            return _mocked_calls
+
+        with patch("pathlib.Path.iterdir", return_value=target_contents), patch(
+            "pathlib.Path.cwd", return_value=working_directory
+        ), patch("pathlib.Path.is_dir", side_effect=mock_n_calls([True])):
+            ctx = FakeContext()
+            ctx.obj["root"] = project_root
+            report = MagicMock(verbose=True)
+            collected = black.get_sources(
+                ctx=ctx,
+                src=("./child",),
+                quiet=False,
+                verbose=True,
+                include=DEFAULT_INCLUDE,
+                exclude=None,
+                report=report,
+                extend_exclude=None,
+                force_exclude=None,
+                stdin_filename=None,
+            )
+        assert not any(
+            mock_args.args[1].startswith("is a symbolic link that points outside")
+            for mock_args in report.path_ignored.call_args_list
+        ), "A symbolic link was reported."
+        report.path_ignored.assert_called_once_with(
+            Path("child", "b.py"), "matches a .gitignore file content"
+        )
+
     def test_report_verbose(self) -> None:
         report = Report(verbose=True)
         out_lines = []