tests: fix conftest sort to preserve definition order within files

The previous sort key used item.name as tie-breaker, which sorted
tests alphabetically within each file. This broke test_tablets.py
where test_tablets_invalidation_decommission_non_cc_node (a
destructive test that decommissions a node) was moved before
test_tablets_invalidation_drop_ks, causing the latter to fail.

Use the original collection index as tie-breaker instead, preserving
the definition order inside each file while still grouping modules
by cluster topology across files.

Author: mykaul

tests/integration/standard/conftest.py

@@ -40,10 +40,17 @@
 
 
 def pytest_collection_modifyitems(items):
-    """Sort tests so modules with the same cluster topology are adjacent."""
+    """Sort tests so modules with the same cluster topology are adjacent.
+
+    Uses the original collection index as tie-breaker so that the
+    definition order inside each file is preserved (important for tests
+    that depend on running order, e.g. destructive tablet tests).
+    """
+    orig_order = {id(item): idx for idx, item in enumerate(items)}
+
     def _sort_key(item):
         module_name = item.module.__name__.rsplit(".", 1)[-1]
-        return (_MODULE_CLUSTER_ORDER.get(module_name, 99), item.fspath, item.name)
+        return (_MODULE_CLUSTER_ORDER.get(module_name, 99), item.fspath, orig_order[id(item)])
 
     items[:] = sorted(items, key=_sort_key)