move code to boot data module

spring-boot-data/src/test/java/com/baeldung/jsonexception/MainControllerIntegrationTest.java

@@ -0,0 +1,19 @@
+package com.baeldung.jsonexception;
+
+import org.junit.Test;
+
+import com.baeldung.jsonexception.CustomException;
+import com.baeldung.jsonexception.MainController;
+
+public class MainControllerIntegrationTest {
+
+    @Test(expected = CustomException.class)
+    public void givenIndex_thenCustomException() throws CustomException {
+
+        MainController mainController = new MainController();
+
+        mainController.index();
+
+    }
+
+}