qa: Check scantxoutset result against gettxoutsetinfo (João Barbosa)
rpc: Improve scantxoutset response and help message (João Barbosa)
Pull request description:
The new response keys `height` and `bestblock` allow the client to know at what point the scan took place.
The help message now has all the response keys (`result` and `txouts` were missing) and it's improved a bit. Note that `searched_items` key is renamed to `txouts`, considering `scantxoutset` is marked experimental.
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Backport of Core PR16285