My name is Erik and I'm a Senior Analyst Programmer for more than 15 years.
The answer is yes, you should ask "outsiders" to test your development work. They will approach the testing more in a "Customer" usage mode than from a project management/developement stand point.
More you invest in testing, less bugs will show up in the future. True Story!
I have about 15 years experience in software development, out of which 5 have been in managing quality assurance processes.
Starting on the premise that you would outsource testing to a proven quality assurance company, I would say that YES, it's a great idea to have outsiders test your code. And I am saying this because of 2 main reasons:
1. You will get that fresh look on things which your testers (as senior as they may be) will lose after the first several iterations. Our mind is wired in such a way that we avoid the "dangerous" areas without even knowing it. We get so stuck in our own ways of doing things that we don't even think there may be other ways of doing it. An "outsider" will not be contaminated by this routine.
2. Outsourcing this will force you to be more organised with the specs and acceptance criteria, thus helping you in other areas such as planning or expectations management.
Hi, I am a CTO for a couple of startups and have worked with in-house testing teams and also outsource testing teams.
Testing in general, is based on the type of user testing you would like to do. If it consists of user-ability , simple flow testing then I guess you can simply run it between some of the existing users, family and friends.
If you want to do special testing like security, load testing then there are companies like bugcrowd.com, 99tests.com which can help you with it.
I can also help with proper suggestions if we can have a follow up call.