Add stream-to-stream join test cases

[viliam-durina]

This line: https://github.com/hazelcast/hazelcast/blob/48fdbaecdc4a0358daf2a2333af159f6f189d253/hazelcast-sql/src/main/java/com/hazelcast/jet/sql/impl/processors/StreamToStreamJoinP.java#L237 Is not covered in the tests. It's rather important and one might be tempted to remove it, and no test will fail.