Cleanup various headers to reduce compilation times (#6255)

* Cleanup various headers to reduce compilation times

src/unittest/test.cpp

@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "itemdef.h"
 #include "gamedef.h"
 #include "mods.h"
+#include "util/numeric.h"
 
 content_t t_CONTENT_STONE;
 content_t t_CONTENT_GRASS;

src/unittest/test_connection.cpp

@@ -24,6 +24,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "settings.h"
 #include "util/serialize.h"
 #include "network/connection.h"
+#include "network/networkpacket.h"
 
 class TestConnection : public TestBase {
 public: