Will I be OK tesing the loop to check the rad for leaks using normal tap water?
You will be perfectly ok using tap water to test for leaks in the rad, though it is usually a good idea to do a final flush with de-ionized or distilled before the tap water drys out so that the minerals in tap water don't stay in the rad when the water evaporates.
However, it always amuses me that people seem to think tap water is somehow poisonous and should never come into contact with any part of a loop. The reality is that the only problems with tap water is that in contains a tiny amount of disolved calcium carbonate and other minerals, possibly some organic material and is conductive.
Minerals in tap water can eventually come out of the fluid and form a film on the blocks/rad etc, but to my mind this will only become a serious problem if you are constantly topping up with fresh tap water such that the net amount of minerals builds up and up. This is how your kettle scales up, i.e. the scale builds up only because of repeated exposure to fresh water. If you repeatedly boiled the same kettle full of water some scale would form on the element but then it would stop once the minerals are used up. Its not like your blocks will end up like your kettle element from having one litre of tap water in the loop for a year. Thats said, since de-ionised is so readily available there is no excuse not to use it.
Organic compounds can exist in de-ionized as well as tap so you still have to use biocide or silver kill coils for either. Distilled is touted as being organic free, as long as you make sure no dust or other matter gets in the loop while you are filling it.
Non conductuvity of certain fluids is only supposed to last for so long anyway and for me is redundant anyway as you surely should be making sure your loop is pretty leakproof form the start.
