CalloutSampleTest.cls

@IsTest
private class CalloutSampleTest {
    
    public class CalloutMock implements HttpCalloutMock {
        public HttpResponse respond(HttpRequest req) {
            return new HttpResponse();
        }
    }
    
    @IsTest
    static void test() {
        System.runAs([select Id from User where Id = :UserInfo.getUserId()][0]) {
            String name = EncodingUtil.convertToHex(Crypto.generateAesKey(128));
            insert new User(
                LastName = name,
                Username = name + '@example.com',
                Email = name + '@example.com',
                ProfileId = UserInfo.getProfileId(),
                Alias = name.left(8),
                TimeZoneSidKey = 'Asia/Tokyo',
                LocaleSidKey = 'ja_JP',
                EmailEncodingKey = 'UTF-8',
                LanguageLocaleKey = 'ja'
            );
        }
        
        Test.startTest();
        Test.setMock(HttpCalloutMock.class, new CalloutMock());
        HttpRequest req = new HttpRequest();
        req.setMethod('GET');
        req.setEndpoint('https://login.salesforce.com');
        new Http().send(req);
        Test.stopTest();
    }

}

修正されました