Added readResource() method for loading files needed for mocking.

src/test/java/org/gitlab4j/api/JsonUtils.java

 package org.gitlab4j.api;
 
+import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.util.List;
@@ -61,4 +62,19 @@ public class JsonUtils {
         System.out.println(jacksonJson.getObjectMapper().writeValueAsString(obj));
         System.out.flush();
     }
+
+    static String readResource(String filename) throws IOException {
+
+        InputStreamReader reader = new InputStreamReader(GitLabApi.class.getResourceAsStream(filename));
+        StringBuilder stringBuilder = new StringBuilder();
+
+        try (BufferedReader br = new BufferedReader(reader)) {
+            String line;
+            while ((line = br.readLine()) != null) {
+                stringBuilder.append(line).append("\n");
+            }
+        }
+
+        return (stringBuilder.toString());
+    }
 }