Add pytest setup and configuration tests

Configure pytest in pyproject.toml to import the repo copy, add a conftest that keeps tests out of the journal and desktop notifications, and test reading the config file and expanding paths.

tests/conftest.py

@@ -0,0 +1,15 @@
+"""Shared fixtures for the simple_backup tests"""
+
+import pytest
+
+from simple_backup import simple_backup as sb
+
+
+@pytest.fixture(autouse=True)
+def no_side_effects(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep tests out of the systemd journal and the desktop notifications"""
+
+    if hasattr(sb, 'j_handler'):
+        sb.logger.removeHandler(sb.j_handler)
+
+    monkeypatch.setattr(sb, '_notify', lambda text: None)