validate-tags: set default for the --ignore option and add tests (#503)

* validate-tags: set default for the --ignore option

When `--ignore` is not passed, the None default causes `Exception`s down
the line when the code tries to iterate over it.

Fixes: 015f935

* add unit tests for validate-tags-file subcommand

* tests: use sorted() for comparisons

The underlying codes relies on sets which apparently don't have
consistent sort order across Pythons.

src/antsibull/cli/antsibull_build.py

@@ -377,7 +377,8 @@ def parse_args(program_name: str, args: list[str]) -> argparse.Namespace:
     validate_tags_shared = argparse.ArgumentParser(add_help=False)
     validate_tags_shared.add_argument(
         '-I', '--ignore',
-        action='append', help='Ignore these collections when reporting errors.'
+        action='append', help='Ignore these collections when reporting errors.',
+        default=[],
     )
     validate_tags_shared.add_argument(
         '--ignores-file',

tests/conftest.py

@@ -0,0 +1,15 @@
+# Copyright (C) 2023 Maxwell G <maxwell@gtmx.me>
+# SPDX-License-Identifier: GPL-3.0-or-later
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or
+# https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from pathlib import Path
+
+import pytest
+
+HERE = Path(__file__).resolve().parent
+TEST_DATA = HERE / 'test_data'
+
+@pytest.fixture
+def test_data_path() -> Path:
+    return TEST_DATA