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.

*/
    std::multimap<CBlockIndex*, CBlockIndex*> m_blocks_unlinked;
    /**
     * Load the blocktree off disk and into memory. Populate certain metadata
     * per index entry (nStatus, nChainWork, nTimeMax, etc.) as well as peripheral
     * collections like setDirtyBlockIndex.
     *
     * @param[out] block_index_candidates  Fill this set with any valid blocks for
     *                                     which we've downloaded all transactions.
     */
    bool LoadBlockIndex(
        const Consensus::Params& consensus_params,
        CBlockTreeDB& blocktree,
        std::set<CBlockIndex*, CBlockIndexWorkComparator>& block_index_candidates)
        EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    
    void Unload() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    CBlockIndex* AddToBlockIndex(const CBlockHeader& block) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    
    CBlockIndex* InsertBlockIndex(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    /**
     * If a block header hasn't already been seen, call CheckBlockHeader on it, ensure
     * that it doesn't descend from an invalid block, and then add it to m_block_index.
     */
    bool AcceptBlockHeader(
        const CBlockHeader& block,
        BlockValidationState& state,
        const CChainParams& chainparams,
        CBlockIndex** ppindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
};