`ScanForWalletTransactions()` has the following comment about its return value:

```
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
```

This is not strictly true. If `abortrescan` is called, then `ScanForWalletTransactions()` will return `nullptr`, even though the scan wasn't successful to the tip.

This in turn causes a minor bug in `importmulti`, where the call doesn't return the correct error and warning about potentially not finding transactions if `abortrescan` was called.

I think the correct fix is for `ScanForWalletTransactions()` to inform the caller that the rescan was aborted.
{
    LOCK(wallet.cs_wallet);
    wallet.AddKeyPubKey(key, key.GetPubKey());
}
BOOST_FIXTURE_TEST_CASE(rescan, TestChain100Setup)
{
    auto chain = interfaces::MakeChain();
    
    const CBlockIndex* const null_block = nullptr;
    CBlockIndex* oldTip = chainActive.Tip();
    GetBlockFileInfo(oldTip->GetBlockPos().nFile)->nSize = MAX_BLOCKFILE_SIZE;
    CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
    CBlockIndex* newTip = chainActive.Tip();
    auto locked_chain = chain->lock();
    
    
    {
        CWallet wallet(*chain, WalletLocation(), WalletDatabase::CreateDummy());
        AddKey(wallet, coinbaseKey);
        WalletRescanReserver reserver(&wallet);