Add check-yaml-filename-extension hook

tests/check_yaml_filename_extension_test.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+import pytest
+
+from pre_commit_hooks.check_yaml_filename_extension import main
+from pre_commit_hooks.util import cmd_output
+
+
+@pytest.mark.parametrize(
+    ('filename', 'new_filename', 'expected_retval'), (
+        ('file_1.yml', 'file_1.yaml', 1),
+        ('.file_2.yml', '.file_2.yaml', 1),
+        ('file_3.yaml', 'file_3.yaml', 0),
+    ),
+)
+def test_main(temp_git_dir, filename, new_filename, expected_retval):
+    with temp_git_dir.as_cwd():
+        temp_git_dir.join(filename).write('---')
+        cmd_output('git', 'add', filename)
+
+        retv = main([filename])
+
+        assert retv == expected_retval
+        assert temp_git_dir.join(new_filename).exists()
+
+        assert retv == 0 or not temp_git_dir.join(filename).exists()