When testing a transaction for mempool acceptance, the fee must be calculated anyway. Thus, with minimal complexity overhead, the testmempoolaccept RPC could return the fee to the user.

#### Useful skills:

Basic understanding of the mempool, the Bitcoin Core RPC interface, and the functional test framework.

#### Want to work on this issue?

The purpose of the `good first issue` label is to highlight which issues are suitable for a new contributor without a deep understanding of the codebase.

You do not need to request permission to start working on this. You are encouraged to comment on the issue if you are planning to work on it. This will help other contributors monitor which issues are actively being addressed and is also an effective way to request assistance if and when you need it.

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

class BlockValidationState;
class CBlockIndex;
class CBlockTreeDB;
class CBlockUndo;
class CChainParams;
class CInv;
class CConnman;
class CScriptCheck;
class CBlockPolicyEstimator;
class CTxMemPool;
class ChainstateManager;
class TxValidationState;
struct ChainTxData;
struct DisconnectedBlockTransactions;
struct PrecomputedTransactionData;
struct LockPoints;
static const unsigned int DEFAULT_MIN_RELAY_TX_FEE = 1000;
static const unsigned int DEFAULT_ANCESTOR_LIMIT = 25;
static const unsigned int DEFAULT_ANCESTOR_SIZE_LIMIT = 101;
static const unsigned int DEFAULT_DESCENDANT_LIMIT = 25;
static const unsigned int DEFAULT_DESCENDANT_SIZE_LIMIT = 101;
static const unsigned int DEFAULT_MEMPOOL_EXPIRY = 336;
static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; 
static const int MAX_SCRIPTCHECK_THREADS = 15;
static const int DEFAULT_SCRIPTCHECK_THREADS = 0;