refactor(assets-controllers): Simplify NftController tests

The `NftController` tests now use mocks instead of real
`AssetsContractController` and `PreferencesController` instances. This
should make the tests easier to read, and it further decouples them
from those controllers.

Additionally, the `setupController` helper method has been updated to
accept a partial set of controller options rather than custom options.

Relates to #3708