refactor: use SpanReader in TestBlockAndIndex

`TestBlockAndIndex` still deserialized its fixed block fixture through `DataStream` and appended a dummy byte to avoid compaction after full consumption.

Use `SpanReader` for that fixture instead.
This removes the leftover dummy-byte workaround and reads the immutable fixture through a read-only view.

src/bench/rpc_blockchain.cpp

@@ -31,10 +31,7 @@ struct TestBlockAndIndex {
 
     TestBlockAndIndex()
     {
-        DataStream stream{benchmark::data::block413567};
-        std::byte a{0};
-        stream.write({&a, 1}); // Prevent compaction
-
+        SpanReader stream{benchmark::data::block413567};
         stream >> TX_WITH_WITNESS(block);
 
         blockHash = block.GetHash();