Add test for the use case of Trait with mapping and callable

[kitchoi]

Motivated by #1258, this PR adds a few tests that exercise the scenario where migration from TraitMap to Map is hard.

The difficulty here is that Trait can take a number of things, and when a mapping and a callable are given to it, it makes the trait a mapped trait because there is a mapping. And when the callable is used for validation (and transformation), both the key value and the mapped value are the same. But when the callable fails, the mapping is used and the key value is not the same as the mapped value.

If we migrate to Map, then the key value would be different from the mapped value. If we use Union to combine them together, then the mapped trait is gone.

I still haven't figured out a migration solution for this scenario that does not involve just reimplement a TraitType that does all these things, but maybe that's what downstream projects have to do if they need to keep these strange behaviour alive, so that Traits can deprecate TraitMap and Trait.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[mdickinson]

LGTM; a couple of nitpicks.