I will ask you, as you have always presented yourself as reasonable.
I try, but am not above my own moments of unreasonableness.
If man has not devised a test for a potential explanation of a phenomenon does this make the potential explanation invalid?
Depends on what you mean by "invalid". If you mean "no true", then explanations can certainly be true even if they cannot be tested. If you mean "not scientific", then again it depends on whether the explanation is _possible_ of being tested. There are explanations that are not currently testable, but could in theory be tested if we could solve some basic problems (enough money, better equipment, more energy, etc). On the other hand, there are explanations that are not even in principle testable.
Teleological explanations are not testable using mechanistic methods. Since science relies entirely on mechanics to test theories, it cannot, in principle, test teleological explanations. ID is a teleological explanation as far as I can tell since it deals exclusively with a "who" behind the mechanics and by admission of its own adherents is not at all interested in mechanics. Therefore, I cannot see, and no one has ever proposed, any way for it to be tested even in theory.
As far as I can tell after having studied this matter with some diligence, this is true for all teleological explanations. If you can point to any argument that explains how teleological explanations can be tested using mechanistic methods I will be more than willing to look at it.