test: Verify findCommonAncestor always initializes outputs

Also add code comment to clarify surprising code noted by practicalswift
https://github.com/bitcoin/bitcoin/pull/18657#issuecomment-614278450

src/interfaces/chain.cpp

@@ -275,6 +275,8 @@ public:
         const CBlockIndex* block1 = LookupBlockIndex(block_hash1);
         const CBlockIndex* block2 = LookupBlockIndex(block_hash2);
         const CBlockIndex* ancestor = block1 && block2 ? LastCommonAncestor(block1, block2) : nullptr;
+        // Using & instead of && below to avoid short circuiting and leaving
+        // output uninitialized.
         return FillBlock(ancestor, ancestor_out, lock) & FillBlock(block1, block1_out, lock) & FillBlock(block2, block2_out, lock);
     }
     void findCoins(std::map<COutPoint, Coin>& coins) override { return FindCoins(m_node, coins); }