Add initial tests for convert

tests/test_convert.py:
Add initial tests for the convert module by adding a parametrized test
for `_files_data_to_dict()`.

tests/test_convert.py

+import io
+from contextlib import nullcontext as does_not_raise
+from os.path import dirname, join, realpath
+from typing import ContextManager
+
+from pytest import mark, raises
+
+from repo_management import convert
+
+RESOURCES = join(dirname(realpath(__file__)), "resources")
+
+
+@mark.parametrize(
+    "file_data, expectation",
+    [
+        ("%FILES%\nusr/\nusr/lib/\n", does_not_raise()),
+        ("%FILES%usr/\nusr/lib/\n", raises(RuntimeError)),
+        ("usr/\nusr/lib/\n", raises(RuntimeError)),
+        ("usr/%FILES%\nusr/lib/\n", raises(RuntimeError)),
+    ],
+)
+def test__files_data_to_dict(
+    file_data: str,
+    expectation: ContextManager[str],
+) -> None:
+    with expectation:
+        convert._files_data_to_dict(data=io.StringIO(file_data))