This makes sure we build the correct variables for testing
translations.

When translating, we could be specifying the variables in different
forms for each id:

- In the singular we could be using a `%{hash}` interpolation
- In the plural we could be using a `%d` interpolation

This changes the tests to accommodate for that: We now use the
variables used in the relevant translation id as the source for the
variables we mix in in specs.