No Apex test class named 'AnimalLocatorTest' was found.

public class AnimalLocator {
    public static String getAnimalNameById(Integer ID) {
        String animal = ' ';
        Http http = new Http();
        HttpRequest request = new HttpRequest();
        String s = string.valueOf(ID);
        request.setEndpoint('https://th-apex-http-callout.herokuapp.com/animals/'+ ID);
        request.setMethod('GET');
        HttpResponse response = http.send(request);
        Map<String,Object> animals = new Map<String,Object>();
        if(response.getStatusCode() == 200) {
            Map<String,Object> results = (Map<String,Object>)JSON.deserializeUntyped(response.getBody());
            animals = (Map<String,Object>) results.get('animal');         
            animal = String.valueOf(animals.get('name'));
        } else {
            system.debug(response.getBody());
        }
        Return animal;
    } 
}

AnimalLocatorMock Class:

@isTest
global class AnimalLocatorMock implements HttpCalloutMock {
    // Implement this interface method
    global HTTPResponse respond(HTTPRequest request) {
        // Create a fake response
        HttpResponse response = new HttpResponse();
        response.setHeader('Content-Type', 'application/json');
        response.setBody('{"animal":{"id":1,"name":"chicken","eats":"chicken food","says":"cluck cluck"}}');
        response.setStatusCode(200);
        return response;
    }
}

AnimalLocatorTest Class:

@isTest
private class AnimalLocatorTest {
    @isTest
    static void testGetCallout() {
        Test.setMock(HttpCalloutMock.class, new AnimalLocatorMock());
        String animalName = AnimalLocator.getAnimalNameById(1);
        system.debug('AnimalName : ' + animalName);
        System.assertEquals(animalName, 'chicken');
    }
}