message: test/util: Use existing chainman in ::PrepareBlock
diff: @@ -74,12 +74,12 @@ CTxIn MineBlock(const NodeContext& node, const CScript& coinbase_scriptPubKey)
  std::shared_ptr<CBlock> PrepareBlock(const NodeContext& node, const CScript& coinbase_scriptPubKey)
  {
  auto block = std::make_shared<CBlock>(
  - BlockAssembler{::ChainstateActive(), *Assert(node.mempool), Params()}
  + BlockAssembler{Assert(node.chainman)->ActiveChainstate(), *Assert(node.mempool), Params()}
  .CreateNewBlock(coinbase_scriptPubKey)
  ->block);
  LOCK(cs_main);
  - block->nTime = ::ChainActive().Tip()->GetMedianTimePast() + 1;
  + block->nTime = Assert(node.chainman)->ActiveChain().Tip()->GetMedianTimePast() + 1;
  block->hashMerkleRoot = BlockMerkleRoot(*block);
  return block;
  