Test prompting during upgrade when a new input is added with a default

[drevell]

This tests an edge case where:

1. A template is installed that has no inputs.
2. The template is updated to add an input that has a default value
3. We run an upgrade
4. The user should be prompted for a value for the new input; the default value should not silently be used.

I thought I saw this last week in my manual testing, but the test passes without changing the logic, so I may have been mistaken.

This change involves some refactoring and test util updates because this is the first time that we've tested prompting logic at the library layer (common/*), rather than the Command layer (commands/*) which wraps the library.