Writing a test class for Salesforce/APEX trigger

I have written down trigger in Salesforce APEX . It is working properly.

Code For Trigger is:

trigger SDRDemoUpdate_test on Event (before update) {

    Map<ID,Event> records = New Map<ID,Event>([Select CreatedBy.Name From Event Where ID In :trigger.new]);

    for (Event obj :Trigger.new){

        obj.SDR_Original_Demo__c = records.get(obj.id).CreatedBy.Name;

    }
}

Now I am trying to write code for its test class. It is giving error on line saying object can not be parsed to String.

Code For Test Class is:

@isTest
 public class originalDemo{
static testMethod void test_original_demo(){
Event obj =new Event();
obj.CreatedBy= 'Tom';
obj.Owner ='Jack';             
obj.What ='Opportunity';

            insert.obj;

userInfo.getName();
     } 

Looking forward to find out the solution. Any help would be appreciated.

Thanks