CalloutUtility.cls

public class CalloutUtility {
	//method to retrieve information about the Salesforce version
    public static HttpResponse performCallout() {
        HttpRequest req = new HttpRequest();
		req.setHeader('Authorization', 'Bearer ' + UserInfo.getSessionID());
		req.setHeader('Content-Type', 'application/json');
		String domainUrl=URL.getSalesforceBaseUrl().toExternalForm();
		system.debug('********domainUrl:'+domainUrl);
		req.setEndpoint(domainUrl+'/services/data/');
		req.setMethod('GET');

		Http h = new Http();
		HttpResponse res = h.send(req);
		system.debug(res.getBody()); 
        return res;
    }
}

CalloutUtilityTest1.cls

@isTest
private class CalloutUtilityTest1 {
    testMethod static void unittest1() {
        StaticResourceCalloutMock mockCallout = new StaticResourceCalloutMock();
        mockCallout.setStaticResource('StaticResourceForCallout');
        mockCallout.setStatusCode(200);
        mockCallout.setHeader('Content-Type', 'application/json');
        
        // Set the mock callout mode
        Test.setMock(HttpCalloutMock.class, mockCallout);
        
        // Call the method that performs the callout
        HTTPResponse res = CalloutUtility.performCallout();
            
        System.assertEquals(200,res.getStatusCode());
        System.assertEquals('application/json', res.getHeader('Content-Type'));   
    }
}

CalloutUtilityTest2

@isTest
private class CalloutUtilityTest2 {
    testMethod static void testCallout() {
        // Set mock callout class 
        Test.setMock(HttpCalloutMock.class, new HTTPMockCallout());
        // from the class that implements HttpCalloutMock. 
        HttpResponse res = CalloutUtility.performCallout();
        
        // Verify response received contains fake values
        String contentType = res.getHeader('Content-Type');
        System.assert(contentType == 'application/json');
        System.assertEquals(200, res.getStatusCode());
    }
}

HTTPMockCallout.cls

@isTest
global class HTTPMockCallout implements HttpCalloutMock {
    global HTTPResponse respond(HTTPRequest req) {
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        string jsonResBody = '{ "label": "Spring \'17",  "url": "/services/data/v39.0","version": "39.0"}';
        res.setBody(jsonResBody);
        res.setStatusCode(200);
        return res;
    }
}