Rollup merge of #91746 - ssomers:btree_tests, r=Mark-Simulacrum

Btree: assert more API compatibility

Introducing a member such as `BTreeSet::min()` would silently break compatibility if no code calls the existing `BTreeSet::min(set)`. `BTreeSet` is the only btree class silently bringing in stable members, apart from many occurrences of `#[derive(Debug)]` on iterators.

r? `@Mark-Simulacrum`