As a very new person in these lists, may I make a comment from my background of writing software for the printing and publishing industry over a period of nearly 40 years..
In the early days, programmers were expected to debug their own code, and usually didn't till it got on site. The customers were then exposed to the nasty underside of software development ie debugging!
As the software industry grew in size it also developed its own professionals and with them came improved testing.
Even today, although I will document my code, I hate testing it!
I always, always insist that part of every contract includes adequate provision for testing with real customer data, but NOT by me.
You should always test software with a third party. That way you get accuracy, thoroughness and impartiality. My 2p Douglas Neale