Require CBlockIndex::IsAssumedValid() to hold cs_main

Summary:
This is a partial backport of [[bitcoin/bitcoin#22932 | core#22932]]
bitcoin/bitcoin@8ef457c

Depends on D13034

Test Plan:
With clang and DEBUG:

`ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13035

src/blockindex.h

@@ -212,7 +212,10 @@ class CBlockIndex {
 
     //! @returns true if the block is assumed-valid; this means it is queued
     //! to be validated by a background chainstate.
-    bool IsAssumedValid() const { return nStatus.isAssumedValid(); }
+    bool IsAssumedValid() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main) {
+        AssertLockHeld(::cs_main);
+        return nStatus.isAssumedValid();
+    }
 
     //! Raise the validity level of this block index entry.
     //! Returns true if the validity was changed.

src/test/validation_chainstatemanager_tests.cpp

@@ -248,7 +248,8 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_activate_snapshot,
                       *chainman.SnapshotBlockhash());
 
     // Ensure that the genesis block was not marked assumed-valid.
-    BOOST_CHECK(!chainman.ActiveChain().Genesis()->IsAssumedValid());
+    BOOST_CHECK(WITH_LOCK(
+        ::cs_main, return !chainman.ActiveChain().Genesis()->IsAssumedValid()));
 
     const AssumeutxoData &au_data =
         *ExpectedAssumeutxo(snapshot_height, ::Params());
@@ -368,6 +369,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_loadblockindex, TestChain100Setup) {
 
     CBlockIndex *validated_tip{nullptr};
     for (int i = 0; i <= cs1.m_chain.Height(); ++i) {
+        LOCK(::cs_main);
         auto index = cs1.m_chain[i];
 
         if (i < last_assumed_valid_idx && i >= assumed_valid_start_idx) {