Test class of Web Service Callouts class..

@isTest
private class WebSvcCalloutTest {
    @isTest static void testEchoString() {            
        // This causes a fake response to be generated
   
        Test.setMock(WebServiceMock.class, new WebServiceMockImpl());
       
        // Call the method that invokes a callout
   
        String output = WebSvcCallout.callEchoString('Hello World!');
       
        // Verify that a fake result is returned
   
        System.assertEquals('Mock response', output);
    }
}