subtype: Remove a bad test comment (#61795)

The comment suggests that the `correct` variable holds the precise
intersection, but this is false. The precise intersection is (as far as
I can tell) not representable. While we're here, add a related
unsoundness that Claude found while looking at this. The common thread
is that these cases involve what Claude calls `F-bounded witnesses`,
i.e. types of the form `Ptr{T} where T<:Ptr`. I haven't read enough of
the literature on F-bounded polymorphism to know if this is a good name
or not, but for want of a better name, let's go with it.