manifolds: Add _test_... methods that check invariants of subset relations #31682
Description
Such as checking that self is a subset of every one of each supersets.
_top_subsets appears to be underspecified.
Code only adds to it but never removes anything, so it is not able to maintain the invariant of being the list of maximal proper subsets of self.
CC: @mjungmath @egourgoulhon @tscrim
Component: manifolds
Issue created by migration from https://trac.sagemath.org/ticket/31682