test: use local CBlockIndex in block read hash mismatch test to avoid data race

Co-authored-by: stickies-v <stickies-v@protonmail.com>

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)