test: Use span for raw data

This change allows to drop brittle sizeof calls in favor of the
std::span::size method.

Other improvements include:

* Use of a namespace to mark test and bench data
* Use of the modern std::byte
* Drop of a no longer used std::vector copy and the bench/data module

src/bench/load_external.cpp

@@ -3,7 +3,7 @@
 // file COPYING or https://www.opensource.org/licenses/mit-license.php.
 
 #include <bench/bench.h>
-#include <bench/data.h>
+#include <bench/data/block413567.raw.h>
 #include <chainparams.h>
 #include <flatfile.h>
 #include <node/blockstorage.h>