This PR extends the AssumeUTXO functional test by submitting a spending transaction for an UTXO that is only available in a the snapshot chainstate (after loading via loadtxoutset), i.e. it hasn't been seen in a block before. With that we can verify that snapshot coins are visible to the mempool.
Note that we unfortunately can't use MiniWallet here, as the only available UTXO to spend from the snapshot chainstate is at height 200, where a P2PKH created from the test framework's deterministic private key is used (see TestNode.generate(...) and the PRIV_KEYS array). Coinbase outputs with smaller heights (<= 199) would be part of the pre-generated chain and hence not qualify for the "UTXO is only in snapshot chainstate and has never been seen in a block" scenario, coinbase outputs with larger heights (>= 201) can't be spent due to immaturity, as the snapshot chainstate block height is 299.
One could of course mine a different chain with outputs that MiniWallet supports (e.g. taproot anyone-can-spend), but this would change the hardcoded AssumeUTXO hash, colliding with other PRs like #28838, so I wanted to avoid that.
