improve choice tests and add downgrade ones

[paulbrauner-da]

The current choice test are not testing much: they are testing that a static call to a v2 choice on a v1 contract will succeed as expected by calling the choice as defined in the v2 template. But no upgrade of the choice argument is performed. This PR changes that. It:

• recognizes that choice arg upgrades only happen in exercise and exercise by key commands, and disable these test cases for all other operations and for "by choice" entry points.
• uses a package name instead of a package ID when exercising choices in commands, which is the only way we can trigger a choice arg upgrade
• removes type annotation from choice arg values

The PR also introduces downgrade tests for choice args.