New prompt testing utility - #7116

This PR implements a more robust way to make assertions about prompts; awkwardy called MockPrompter. It wraps the auto-generated PrompterMock and adds functions for registering expectations and verifying them.

to use it in a test:

pm := prompter.NewMockPrompter(t)
defer pm.Verify()

pm.RegisterSelect("animal type?", []string{"cat", "dog", "moose"}, func(_, _ string, opts []string) (int, error) {
  return prompter.IndexFor(opts, "cat")
}
pm.RegisterInput("cat name?", func(_, d string) (string, error) {
  return d, nil
}
pm.RegisterInput("cat color?", func(_, _ string) (string, error) {
  return "tortoiseshell", nil
}
pm.RegisterConfirm("hug the cat?", func(_ string, _ bool) (bool, error) {
  return true, nil
}

The MockPrompter ensures that:

• unexpected prompts return an intelligible error
• expected prompts are all seen and show up in the order they were registered

and avoids any interface{} shenanigans.

This PR includes updates to two test files to illustrate the new approach.