Before star wars is implemented
The time to consider manned atomic testing in space is now, before star wars is implemented. Since testing might damage some satellite and owners would have to be compensated, if it is done 25 years in the future and star wars is a reality then the military might just decide to stage a war in order to destroy the satellite to free up orbits for testing without having to payout any compensation.
Obviously to fly out of the solar system the explosions would have to be consecutive, but that is a small technical detail. The real stretch of the imagination and faith is that the critical mass can actually be contained in space and well that's what needs testing.
Stephen Hawking suggests that unless we do conquer space, eventually we will destroy ourselves, which is a reality for our children. So however dangerous the project proposal seems, we need now the cold war is over to examine this expensive option very closely. There is no leap in physics required, only the rejection of Einstein's law for being far too clever, rather like socialism or communism.
Before star wars is implemented continued
The only real limiting factor to such testing earlier on in our history has been the maturity of payload performance. No one is going to volunteer to sit on top of an exploding atomic tube unless it has been tested. If it cracks half way then you are not coming back. So bearing in mind a return interstellar voyage may take 1000 explosions, a target benchmarking testing programme may be 100. So even if your payload launch success rate is 98% as it sort of has reached nowadays, 2 out of the 100 will fail and scatter radioactivity on the earth (but not explode).
We should not so easily accept that the speed of light is an impassable barrier. In making business decisions you have to be positive (some of the time). In business no one plans 14 years in the future which is what you are asking an interstellar theorist to do if they accept the theory of relativity. Of course you can always sidetrack yourself into cryogenics and wait for developments...and wait.
So without the cold war the treaties of the mid 60s, prohibiting atomic testing in space is not vital to maintain peace and we sort of have a window before star wars kicks in? The cost then of testing in space is a high risk of serious nuclear conflict because commercial satellites are expensive and it would be a little too tempting to avoid the compensation payments for damages from EMF by engineering a war to avoid the payments.