Well, I don´t know why you would have to do more nuclear tests when you know it can be done in computers. Out of spite?
So in the 1965 war USA put sanctions on both Pakistan and India, but in a future war you think USA would only put sanctions on India? I think the world would put sanctions on both your countries...
But really, a war between India and Pakistan with sanctions put on you. How long do you think such a war would last for? And don´t you think that the Indian armed forces have planned for such an event disregarding any fighter they buy and have spare parts so they can keep fighting til the war is over? (Unless it becomes some WWI war alá Iran-Iraq but that would hardly be the case considering Indias superiority)
We need to spend millions of dollars to build up our labs to simulate it virtually. And we don't have the vast data that other countries have accumulated with their numerous tests to validate various parameters. Hence, for now, a real test is the only way out.
Even if they put sanctions on both countries, we don't want the war to end in a stalemate and perpetuate the pak problem. And last time during Kargil war, Israel and USSR helped us out by supplying us items out of their war reserve. We don't want "friends" like US imposing sanctions during that time.
There might be items which are past their shelf life, or for whatever reasons weren't procured in required numbers, which is entirely possible in Indian scenario given the Indian procurement policy and long delays. There might also be items which though in sufficient quantities run out faster than expected etc.
You buy arms so that it helps you when required. You don't buy it if there is no guarantee of performance when required in actual scenario. Buying from US is like that, you are constantly watching your back!