Have testBlockValidity hold cs_main instead of caller

The goal of interfaces is to eventually run in their own process,
so we can't use EXCLUSIVE_LOCKS_REQUIRED in their declaration.

However TestBlockValidaty will crash (in its call to ConnectBlock)
if the tip changes from under the proposed block.

Have the testBlockValidity implementation  hold the lock instead,
and non-fatally check for this condition.

src/rpc/mining.cpp

@@ -371,8 +371,6 @@ static RPCHelpMan generateblock()
 
     ChainstateManager& chainman = EnsureChainman(node);
     {
-        LOCK(cs_main);
-
         std::unique_ptr<CBlockTemplate> blocktemplate{miner.createNewBlock(coinbase_script, /*use_mempool=*/false)};
         if (!blocktemplate) {
             throw JSONRPCError(RPC_INTERNAL_ERROR, "Couldn't create new block");
@@ -387,8 +385,6 @@ static RPCHelpMan generateblock()
     RegenerateCommitments(block, chainman);
 
     {
-        LOCK(cs_main);
-
         BlockValidationState state;
         if (!miner.testBlockValidity(block, /*check_merkle_root=*/false, state)) {
             throw JSONRPCError(RPC_VERIFY_ERROR, strprintf("testBlockValidity failed: %s", state.ToString()));