uses Python's unittest.mock to patch the Google auth credentials check

tests/test_gcs.py

@@ -1,7 +1,37 @@
+import os
+from unittest.mock import patch
+
 from dapla.gcs import GCSFileSystem
 
 
-def test_instance() -> None:
-    # Chack that instantiation works with the current version of pyarrow
+@patch('google.auth.default', return_value=(None, None))
+def test_instance(mock_auth) -> None:
     client = GCSFileSystem()
     assert client is not None
+
+
+@patch('google.auth.default', return_value=(None, None))
+def test_init_with_dapla_lab(mock_auth) -> None:
+    os.environ["DAPLA_REGION"] = "dapla-lab"
+    client = GCSFileSystem(project="test-project")
+    assert client is not None
+
+
+@patch('google.auth.default', return_value=(None, None))
+def test_init_with_cloud_run(mock_auth) -> None:
+    os.environ["DAPLA_REGION"] = "cloud-run"
+    client = GCSFileSystem(project="test-project")
+    assert client is not None
+
+
+@patch('google.auth.default', return_value=(None, None))
+def test_init_with_custom_region(mock_auth) -> None:
+    os.environ["DAPLA_REGION"] = "custom-region"
+    client = GCSFileSystem(project="test-project")
+    assert client is not None
+
+
+@patch('google.auth.default', return_value=(None, None))
+def test_init_with_additional_kwargs(mock_auth) -> None:
+    client = GCSFileSystem(project="test-project", timeout=100)
+    assert client is not None