Add tests for command line handling and user detection

Test how command line and config values are combined, the exit codes for an incomplete SSH setup and a bad config, and the user detection under sudo/doas (simulated by patching geteuid and the environment).

tests/conftest.py

@@ -1,5 +1,8 @@
 """Shared fixtures for the simple_backup tests"""
 
+import sys
+from typing import Callable
+
 import pytest
 
 from simple_backup import simple_backup as sb
@@ -13,3 +16,15 @@ def no_side_effects(monkeypatch: pytest.MonkeyPatch) -> None:
         sb.logger.removeHandler(sb.j_handler)
 
     monkeypatch.setattr(sb, '_notify', lambda text: None)
+
+
+@pytest.fixture
+def run_cli(monkeypatch: pytest.MonkeyPatch) -> Callable[..., int]:
+    """Run simple_backup() with the given command line arguments"""
+
+    def run(*argv: str) -> int:
+        monkeypatch.setattr(sys, 'argv', ['simple_backup', *argv])
+
+        return sb.simple_backup()
+
+    return run