refactor: Replace fwd-decl with proper include

This is fine, because the span.h include is lightweight and a proper
include will be needed anyway when switching to std::span.

src/test/util/net.h

@@ -12,6 +12,7 @@
 #include <netaddress.h>
 #include <node/connection_types.h>
 #include <node/eviction.h>
+#include <span.h>
 #include <sync.h>
 #include <util/sock.h>
 
@@ -28,9 +29,6 @@
 
 class FastRandomContext;
 
-template <typename C>
-class Span;
-
 struct ConnmanTestMsg : public CConnman {
     using CConnman::CConnman;