Add `UnionArray` tests exercising hashing, group-by, distinct, and aggregates

[friendlymatthew]

Is your feature request related to a problem or challenge?

A follow up from #18718

I added some high-level tests in a separate branch that exercise hashing union arrays through group by, distinct, aggregation queries

Once these 2 upstream PRs in arrow land get merged, we should follow up with a couple of integration tests:

• Add support for Union types in RowConverter arrow-rs#8839
• Add comparison support for Union arrays arrow-rs#8838

Related: #18718 (comment)
cc @adriangb @Jefffrey

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