[wpiutil] Refactor SpanMatcher and TestPrinters from ntcore (#5658)

ntcore/src/test/native/cpp/TestPrinters.cpp

@@ -4,8 +4,6 @@
 
 #include "TestPrinters.h"
 
-#include <wpi/json.h>
-
 #include "Handle.h"
 #include "PubSubOptions.h"
 #include "net/Message.h"
@@ -13,12 +11,6 @@
 #include "networktables/NetworkTableValue.h"
 #include "ntcore_cpp.h"
 
-namespace wpi {
-void PrintTo(const json& val, ::std::ostream* os) {
-  *os << val.dump();
-}
-}  // namespace wpi
-
 namespace nt {
 
 void PrintTo(const Event& event, std::ostream* os) {