test: check_mempool_result negative feerate

[kevkevinpal]

Adds test coverage in mempool_accept.py to check if a negative maxfeerate is input into check_mempool_result
Asserts "Amount out of range" error message and -3 error code

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Type	Reviewers
ACK	maflcko, brunoerg, davidgumberg

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[maflcko]

lgtm ACK bf264e0

[brunoerg]

nice, utACK bf264e0

[davidgumberg]

Looks great, ACK bf264e0

If you need to rebase you could also add a check that the same error is thrown when maxfeerate exceeds MAX_MONEY:

inline bool MoneyRange(const CAmount& nValue) { return (nValue >= 0 && nValue <= MAX_MONEY); }

CAmount AmountFromValue(const UniValue& value, int decimals)
{
    // [...]
    if (!MoneyRange(amount))
        throw JSONRPCError(RPC_TYPE_ERROR, "Amount out of range");
    // [...]
}

The same tests should probably be added for sendrawtransaction but that could be done in a follow up.