* Fix _clean_result for debug callback with 1 char var names
The check in _clean_results was removing any keys that happened
to be one of the chars in ('invocation') instead of the string
'invocation'. This was meant to be a tuple but there was no comma
so the for iterated the string instead of the tuple.
Introduced in 9dba580204
Update unit test to catch this.
Fixes#33723
* Use .pop() to remove invocation from results dict
In base callback _clean_results, simplify the way the
'invocation' item is removed.
Add some more unit tests.
a57d6a4206