test: Use mock for simulating permissioned systems (#102)

The `add_repository_invalid` test can fail on systems where the user has
read permissions to the `/root` directory. (This is likely uncommon, but
not infeasible.)

This adds a mocking function to simulate the `os.access` function such
that it _always_ returns `False` for the `/root` directory. I figure
this is preferable than trying to find a file that will not be
user-readable on every system, especially because our unit tests have no
need to test the behavior of the `os.access` function itself.

Author: mitchnegus

src/firewheel/tests/unit/control/test_repository_db.py

@@ -1,10 +1,25 @@
 
+import os
+from unittest.mock import patch
+
 import pytest
 from pathlib import Path
 
 from firewheel.control.repository_db import RepositoryDb
 
 
+@pytest.fixture
+def mock_permissioned_filesystem():
+    # Save a reference to the original `os.access` method before mocking
+    os_access = os.access
+
+    def _deny_user_access_to_root_home_directory(path, mode, **kwargs):
+        return False if Path(path) == Path("/root") else os_access(path, mode, **kwargs)
+
+    with patch("os.access", side_effect=_deny_user_access_to_root_home_directory):
+        yield
+
+
 def create_test_repo(repo_path):
     repo_path.mkdir()
     return str(repo_path)
@@ -97,7 +112,9 @@ def test_add_repository(self, repository_db, repo_entry):
             [{"path": "asdf"}, FileNotFoundError],  # ----- missing directory
         ],
     )
-    def test_add_repository_invalid(self, invalid_entry, exception, repository_db):
+    def test_add_repository_invalid(
+        self, invalid_entry, exception, mock_permissioned_filesystem, repository_db
+    ):
         with pytest.raises(exception):
             repository_db.add_repository(invalid_entry)