tests/tree_model_iterator: Fix/add const+non tests
- Rename test_const_conversion() to _comparison() as that's what it does
- Rename test_assignment_from_const() to _to_const(), ditto - & fix that
not actually comparing the 2 different constness types, by using auto!
- & make it actually test assignment; it was only trying construction!
- Add real _from_const(), which static_asserts !constructible/assignable