Port tests in `union.rs` to sqllogictest

[alamb]

Is your feature request related to a problem or challenge?

Part of #6195. We are trying to keep the DataFusion code base clean and easy to contribute to and modify. Part of doing so is having a single location for most test coverage so new tests can be added easily and people can follow the existing patterns easily

Describe the solution you'd like

Port the tests in https://github.com/apache/arrow-datafusion/blob/main/datafusion/core/tests/sql/union.rs to union.slt in https://github.com/apache/arrow-datafusion/tree/main/datafusion/core/tests/sqllogictests/test_files

Notes:

1. Instructions for running sqllogitest are here: https://github.com/apache/arrow-datafusion/tree/main/datafusion/core/tests/sqllogictests
2. If you don't know how to port any particular test, please leave it in the .rs or ask on the pull request

Describe alternatives you've considered

No response

Additional context

No response

[alamb]

This test is small -- so I think this is a good first issue