Restore _id() removed by the merge

support_remote_exec_only(), added in main after this branch replaced the
other uses of _id() with combine(), still returns it.  The merge was
clean, but importing test.support failed with a NameError.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/support/__init__.py

@@ -1374,6 +1374,9 @@ def wrapper(self):
 #=======================================================================
 # unittest integration.
 
+def _id(obj):
+    return obj
+
 def mark(label, value=True, /):
     """Add a label to a test method or class.  Use it as a decorator.