I'm very much with JF on his idea of Company Resourse Management. Design and Flight Test must work together from the outset of a project. Such a doctrine should be a culture and not merely enshrined in company procedures. As an aside, I remember a case when Design and Commercial departments didn't think to ask Flight Test whether certain modifications would require Flight Test work. Result, when surprise, surprise, regulatory authority says 'What about the test flying?': inadequate price quoted/contracted and thus significant loss of profit margin.
I agree the problems when there is significant geographical separation: not easy to pursue a working relationship of mutual trustful harmony.
With as expensive a business as ours, we should not have time for petty jealousies: it matters not who comes up with the fix, if it works to prevent costly additional resources and time. That way we should keep the CEO/Shareholders and, most importantly, our customers, happy.