refactor: unify test/util/wallet.h with wallet/test/util.h

files share the same purpose, and we shouldn't have wallet code
inside the test directory.

This later is needed to use wallet util functions in the bench
and test binaries without be forced to duplicate them.

src/bench/wallet_loading.cpp

@@ -7,7 +7,7 @@
 #include <node/context.h>
 #include <test/util/mining.h>
 #include <test/util/setup_common.h>
-#include <test/util/wallet.h>
+#include <wallet/test/util.h>
 #include <util/translation.h>
 #include <validationinterface.h>
 #include <wallet/context.h>