[rest] Test UTXO retrieval when ignoring the mempool
[rest] Test UTXO retrieval when ignoring the mempool

Current REST API always returns empty UTXO when invoked without
`/checkmempool/` URL part.

Cherry-picked from: bitcoin/bitcoin 9cb9af8c
Conflicts: limited scope to only the QA test, as rest.cpp
           implementation is incompatible with Dogecoin 1.14

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