[tests.conftest] Prepare helpers for multi-class database support

tests/conftest.py

@@ -162,10 +162,14 @@ class DatabaseCheckers:
 
             assert len(split[k]) == lengths[k]
             for s in split[k]:
-                assert any(
-                    [s[0].startswith(k) for k in prefixes]
-                ), f"Sample with name {s[0]} does not start with any of the prefixes in {prefixes}"
-                assert s[1] in possible_labels
+                assert any([s[0].startswith(k) for k in prefixes]), (
+                    f"Sample with name {s[0]} does not start with any of the "
+                    f"prefixes in {prefixes}"
+                )
+                if isinstance(s[1], list):
+                    assert all([k in possible_labels for k in s[1]])
+                else:
+                    assert s[1] in possible_labels
 
     @staticmethod
     def check_loaded_batch(