Test Parallel Transaction Builders

I had a suspicion that the wallet would create conflicting transactions and double spends if you used multiple transaction builders at the same time.

This testing pretty thoroughly explores that, and it seems that the wallet does not create conflicting transactions, but is working correctly. Which means the errors in the wild we are seeing are due to some other issue.

This PR only contains a small change to the actual code (first commit), most of it is just testing that didn't fail.