Adds fixture to download atlases for test_validation.py (#498)

tests/atlasgen/conftest.py

@@ -0,0 +1,21 @@
+from pathlib import Path
+
+import pytest
+
+from brainglobe_atlasapi import BrainGlobeAtlas
+
+
+@pytest.fixture(autouse=True)
+def setup_preexisting_local_atlases():
+    """Automatically setup all tests to have three downloaded atlases
+    in the test user data."""
+    preexisting_atlases = [
+        ("example_mouse_100um", "v1.2"),
+        ("allen_mouse_100um", "v1.2"),
+        ("kim_dev_mouse_e11-5_mri-adc_31.5um", "v1.3"),
+    ]
+    for atlas_name, version in preexisting_atlases:
+        if not Path.exists(
+            Path.home() / f".brainglobe/{atlas_name}_{version}"
+        ):
+            _ = BrainGlobeAtlas(atlas_name)