The members which are currently hidden may get removed from `package:test_api` because they have no uses.
https://github.com/dart-lang/test/pull/2030

Switch to a `show` and list all the exported APIs. This is currently a no-op change, and is forward compatible with the breaking change when it lands.

A `show` is also safer than a `hide` when exporting across package boundaries. Adding a new member to a library _can be_ breaking, but is often treated as non-breaking. If a new member is added to `package:test_api/scaffolding.dart` it will need to be manually added to the list to be usable from `package:flutter_test`, but we anyways want to add a CHANGELOG entry for user facing changes.