PubSubTableProvider tries to set windows after GroupByKey

[damccorm]

A SqlTransform may contain a GroupByKey in the global window, this prevents setting windows afterwards. The PubSubTableProvider tries anyway.

This was exposed by a INSERT INTO pubsub VALUES (1), (2), which implicitly becomes INSERT INTO pubsub SELECT 1 UNION SELECT 2. This then becomes a GroupByKey union.

For now I'm going to simplify the test.

Imported from Jira BEAM-12853. Original Jira may contain additional context.
Reported by: apilloud.