Add tests for simplifying multiple aggregate expressions

[alamb]

Which issue does this PR close?

• Part of Make Clickbench Q29 5x faster for datafusion by extracting SUM(..) clauses #15524
• Related to Rewrite SUM(expr + scalar) --> SUM(expr) + scalar*COUNT(expr)  #20749

Rationale for this change

As part of #15524 I am working on some optimizations for queries with multiple aggregates. To make it clear what is changing, and ensure I don't introduce regressions, I want to add the tests to main first.

Merging the tests first should also make #20749 easier to review

What changes are included in this PR?

1. Add new tests

Are these changes tested?

Yes, all tests

Are there any user-facing changes?

No

[adriangb]

let's 🚢 it!

[alamb]

Thanks @adriangb