Consolidate tests for unparser / plan to sql to make them easier to find

[alamb]

          Since this is testing `plan_to_sql` it might make more sense to put it with the rest of the tests for plan_to_sql:  https://github.com/apache/datafusion/blob/main/datafusion/sql/tests/sql_integration.rs#L4669

However, the fact that those tests don't have "plan_to_sql" in their name is confusing too.

Maybe we could consolidate the tests into something like datafusion/sql/tests/sql_integration/plan_to_sql.rs 🤔 (as a follow on PR)

Originally posted by @alamb in #10612 (comment)