Merge bitcoin/bitcoin#29055: tests, bench: Fix issue with CWallet::LoadWallet() being called in the wrong places

bd7f5d33e3 wallet: Assert that the wallet is not initialized in LoadWallet (Andrew Chow)
fb0b6ca4e5 tests, bench: Remove incorrect LoadWallet() calls (Andrew Chow)

Pull request description:

  `CWallet::LoadWallet()` expects to be called after a `CWallet` is constructed, but before any of its member functions called. Doing so invalidates pointers which causes issues with some PRs and branches that I am working on. This was being used incorrectly in a few tests and benchmarks, resulting in segfaults.

  As a precaution for this kind of issue in the future, I've also added a few asserts to `LoadWallet()` so that developers will notice when it is used incorrectly.

  As similar issue was fixed in #27666

ACKs for top commit:
  S3RK:
    ACK bd7f5d33e3
  furszy:
    ACK bd7f5d33

Tree-SHA512: 7664f12b8452994e7fc4d7d4f77697fb5f75edb0dba95ba99a4a23ec03d5b8e0ecbdcb7635547a0e8b4f89f708f98dcb5d039df0559e24b1ae411ed630e16e14

src/wallet/test/util.cpp

@@ -24,7 +24,6 @@ std::unique_ptr<CWallet> CreateSyncedWallet(interfaces::Chain& chain, CChain& cc
         LOCK2(wallet->cs_wallet, ::cs_main);
         wallet->SetLastBlockProcessed(cchain.Height(), cchain.Tip()->GetBlockHash());
     }
-    wallet->LoadWallet();
     {
         LOCK(wallet->cs_wallet);
         wallet->SetWalletFlag(WALLET_FLAG_DESCRIPTORS);