Improve test coverage for FormattingHelper#format_money
Improve test coverage for FormattingHelper#format_money

I believe that the origin of the code in `FormattingHelper#format_money` was
from the code which is now in
`SmartAnswer::I18nRenderer#value_for_interpolation`. Thus at least in some
scenarios I would expect a `SmartAnswer::Money` object to be passed in rather
than a `String`. This commit adds assertions for the former.

[1]: https://github.com/alphagov/smart-answers/blob/7e26c554be106b2888f44bf92746e5fc7f6d819f/lib/smart_answer/i18n_renderer.rb#L33-L34

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