Merge bitcoin/bitcoin#33154: test: use local CBlockIndex in block read hash mismatch check

cb173b8 test: use local `CBlockIndex` in block read hash mismatch test to avoid data race (Lőrinc)

Pull request description:

  Avoid mutating the shared active tip `CBlockIndex` in the `blockmanager_readblock_hash_mismatch` test.
  Instead, construct a local `CBlockIndex` with only the required fields set, ensuring the test remains self-contained and hopefully eliminating the data race reported in bitcoin/bitcoin#33150.

ACKs for top commit:
  stickies-v:
    ACK cb173b8
  maflcko:
    lgtm ACK cb173b8

Tree-SHA512: 790528db0659f8cc5b87ed2b316bf274af68edc6158b0ce8821baccddf8d9bc4074afcb7260e3a61d5013d24ab51cc5c31e36693b8fb5ab913a44229fd6ad36b

Author: fanquake

src/test/blockmanager_tests.cpp

@@ -139,12 +139,18 @@ BOOST_FIXTURE_TEST_CASE(blockmanager_block_data_availability, TestChain100Setup)
 
 BOOST_FIXTURE_TEST_CASE(blockmanager_readblock_hash_mismatch, TestingSetup)
 {
-    CBlockIndex* fake_index{WITH_LOCK(m_node.chainman->GetMutex(), return m_node.chainman->ActiveChain().Tip())};
-    fake_index->phashBlock = &uint256::ONE; // invalid block hash
+    CBlockIndex index;
+    {
+        LOCK(cs_main);
+        const auto tip{m_node.chainman->ActiveTip()};
+        index.nStatus = tip->nStatus;
+        index.nDataPos = tip->nDataPos;
+        index.phashBlock = &uint256::ONE; // mismatched block hash
+    }
 
     ASSERT_DEBUG_LOG("GetHash() doesn't match index");
-    CBlock dummy;
-    BOOST_CHECK(!m_node.chainman->m_blockman.ReadBlock(dummy, *fake_index));
+    CBlock block;
+    BOOST_CHECK(!m_node.chainman->m_blockman.ReadBlock(block, index));
 }
 
 BOOST_AUTO_TEST_CASE(blockmanager_flush_block_file)