Write tests for streaming_merge

[kazuyukitanimura]

Is your feature request related to a problem or challenge?

With #6874, physical_plan::merge::streaming_merge is now public.

Without some care, the change may be lost in some future refactoring (e.g. if the code is moved to another module, for example, that is not pub).

Describe the solution you'd like

Write a test, perhaps as an example of how to use this API, in: https://github.com/apache/arrow-datafusion/tree/main/datafusion-examples/examples

Might be able to adapt some of the existing code in https://github.com/apache/arrow-datafusion/blob/main/datafusion/core/tests/fuzz_cases/merge_fuzz.rs or https://github.com/apache/arrow-datafusion/blob/main/datafusion/core/benches/sort.rs

Describe alternatives you've considered

No response

Additional context

No response

[viirya]

Yea, it is good to add test for public API.