NOTE: The code provided is an example. You'll need to review and make modifications for your organization.
 

@isTest
private class FeedCommentTest {
    @isTest static void testFeedComment() {
        
        User usr = new User();
        usr.ProfileId = [SELECT Id FROM Profile WHERE Name != 'System Administrator' limit 1].Id;// replace this with the profile name associated with id 00e6w000000QHyUAAW             
        usr.LastName = 'Test';
        usr.Email = 'test@test.com';
        usr.Username = 'test@test.com' + System.currentTimeMillis();
        usr.CompanyName = 'Salesforce';
        usr.Title = 'Title';
        usr.Alias = 'Roger';
        usr.TimeZoneSidKey = 'America/Los_Angeles';
        usr.EmailEncodingKey = 'UTF-8';
        usr.LanguageLocaleKey = 'en_US';
        usr.LocaleSidKey = 'en_US';
        
        System.runAs(usr) {
            
            //Parent Record
            Account acc = new Account(Name = 'Test Account');
            insert acc;
            
            Sales_Engineering_Request__c ser = new Sales_Engineering_Request__c(); 
            //populate required fields of this object and perform insert
            insert ser;
            
            //Create Related Feed Item Record
            FeedItem fi = new FeedItem(ParentId = ser.Id, Body = 'Test Body');
            insert fi;
            
            //Create Feed Comment Record
            FeedComment fc = new FeedComment(FeedItemId = fi.Id, parentId = ser.id, CommentBody = 'Test Comment');
            insert fc;
            
            //Get Feed Comment Record
            FeedComment objFC = [Select Id, CommentBody, FeedItemId, ParentId FROM FeedComment LIMIT 1];
            
            //Check Feed Comment Parent Id
            System.assertEquals(objFC.ParentId, ser.Id);
        } 
    }   
}

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