In OTL, the early (pre-1970) Nuclear Powers had pieces of flat empty (by the standards of the Great Power) land they controlled and could do Nuclear testing in. US:New Mexico/Nevada, UK: Central Australia, France: Algeria, USSR (A bounty of choices) and the PRC (Xingjiang (sp?) in NW China)
However, we've had a number of Alternate Great Powers that would have had considerably more difficulty.
In a Central Powers victory, while German and AH would have had access to more of Europe, I'm not sure anywhere East of the Urals that they control would have been as good as Nevada or Australia (*maybe Karelia, but even that's much more populated than the places iOTL. And I have *no* confidence that in a CP victory work either a) that the Ottomans remain a tight enough ally to do Nuclear development work there, or b) that testing in the Saudi desert would be that great as well.
In the case of China, if you had shown a 19th Century Chinaman a China without Tibet or Xingjiang (make the border in the west run from the northern tip of OTL Burma to the southern tip of OTL Mongolia), they would still expect the remaining China to be a World Power, and probably wouldn't feel revanchist about not having them. Where does *that* power test nukes, Manchuria???
I have similar feelings toward a South Asian world Power or for that matter a late 19th/early 20th century Brazil wank...
While Ocean based testing (see Bikini) is possible, doing the *initial* test there seems difficult.