Keep the tests away from the real data.txt

Refs #27
Task: MIL-004#5

tests/conftest.py

@@ -0,0 +1,16 @@
+"""Shared pytest configuration for the snake game tests."""
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def isolated_working_directory(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """Run every test in an empty temporary working directory.
+
+    The high score file has a relative default path (`data.txt`). Without this
+    fixture, a test that uses the default would read and write the real file of
+    the repository or of the directory pytest was started from.
+    """
+    monkeypatch.chdir(tmp_path)