Add direct tests for the Windows RNG functions

[RalfJung]

Once rust-lang/rust#121337 lands we'll support 3 RNG functions on Windows. I don't know which of these we are testing at all, since we're only testing them indirectly via std and getrandom. Would be good to have a test that calls them directly, just to make sure that they remain covered.

@ChrisDenton if you could help write such a test that would be great. :)

[ChrisDenton]

I don't really know miri that well but sure I can throw up some some code.

[RalfJung]

Reopening since we should add a test for ProcessPrng once rust-lang/rust#121337 lands.