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ARROW-10663: [C++] Fix is_in and index_in behaviour

* Reject duplicates in SetLookupOptions::value_set, because otherwise the indices returned by index_in would be relative to a deduplicated value_set.

* Honour SetLookup::skip_nulls in is_in.

* Revamp tests.

Closes #9164 from pitrou/ARROW-10663-set-lookup-null

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>