scripted-diff: replace chainActive -> ::ChainActive()

Though at the moment ChainActive() simply references `g_chainstate.m_chain`,
doing this change now clears the way for multiple chainstate usage and allows
us to script the diff.

-BEGIN VERIFY SCRIPT-
git grep -l "chainActive" | grep -E '(h|cpp)$' | xargs sed -i '/chainActive =/b; /extern CChain& chainActive/b; s/\(::\)\{0,1\}chainActive/::ChainActive()/g'
-END VERIFY SCRIPT-

src/test/util.cpp

@@ -84,7 +84,7 @@ std::shared_ptr<CBlock> PrepareBlock(const CScript& coinbase_scriptPubKey)
             .CreateNewBlock(coinbase_scriptPubKey)
             ->block);
 
-    block->nTime = ::chainActive.Tip()->GetMedianTimePast() + 1;
+    block->nTime = ::ChainActive().Tip()->GetMedianTimePast() + 1;
     block->hashMerkleRoot = BlockMerkleRoot(*block);
 
     return block;