log: add LoadBlockIndex() message for assumedvalid blocks

I found this useful during unittest debugging.

src/validation.cpp

@@ -4353,6 +4353,8 @@ bool ChainstateManager::LoadBlockIndex()
                 assert(any_chain([](auto chainstate) { return !chainstate->reliesOnAssumedValid(); }));
 
                 first_assumed_valid_height = block->nHeight;
+                LogPrintf("Saw first assumedvalid block at height %d (%s)\n",
+                        first_assumed_valid_height, block->ToString());
                 break;
             }
         }