Improve pytest warning on missing test data

tests/conftest.py

@@ -34,17 +34,36 @@
 
     from dissect.target.plugin import OSPlugin
 
-# Test if the data/ directory is present and if not, as is the case in Python
-# source distributions of dissect.target, we give an error
-data_dir = absolute_path("_data")
-if not pathlib.Path(data_dir).is_dir():
-    raise pytest.PytestConfigWarning(
-        f"No test data directory {data_dir} found.\n"
-        "This can happen when you have downloaded the source distribution\n"
-        "of dissect.target from pypi.org. If so, retrieve the test data from\n"
-        "the dissect.target GitHub repository at:\n"
-        "https://github.com/fox-it/dissect.target"
-    )
+
+def pytest_sessionstart(session: pytest.Session) -> None:
+    # Test if the _data/ directory is present and if not, as is the case in Python
+    # source distributions of dissect.target, we give an error
+    data_dir = absolute_path("_data")
+    if not data_dir.is_dir():
+        session.shouldfail = (
+            "! !\n"
+            f"No test data directory {data_dir} found.\n"
+            "This can happen when you have downloaded the source distribution\n"
+            "of dissect.target from pypi.org. If so, retrieve the test data from\n"
+            "the dissect.target GitHub repository at:\n"
+            "https://github.com/fox-it/dissect.target"
+            "\n! !"
+        )
+    else:
+        # Test if the test data looks like LFS references and if so, we give an error
+        # This can happen when git-lfs is not installed or not configured correctly
+        for file in (path for path in data_dir.rglob("*") if path.is_file()):
+            with file.open("rb") as fh:
+                if fh.read(42) == b"version https://git-lfs.github.com/spec/v1":
+                    session.shouldfail = (
+                        "! !\n"
+                        "Test data files look like git-lfs references.\n"
+                        "This can happen when git-lfs is not installed or not configured correctly.\n"
+                        "Install git-lfs and run: \n"
+                        "git lfs install && git lfs pull"
+                        "\n! !"
+                    )
+            break
 
 
 @pytest.fixture(autouse=True)