refactor test classes to their own modules

MyJsonString.java

@@ -0,0 +1,14 @@
+package org.json.junit;
+
+import org.json.*;
+
+/**
+ * Used in testing when a JSONString is needed
+ */
+class MyJsonString implements JSONString {
+
+    @Override
+    public String toJSONString() {
+        return "my string";
+    }
+}