Add `spec` coverage for `Refills::ImportGenerator`

Adds test coverage for generating previously defined snippets. Provides a hook
to test future snippet generation.
Haha, I spent about 45 minutes going down that rabbit hole today.

It's defined [here](https://github.com/rails/rails/blob/b2f62f54f560490f337e0c22755922283e4c3557/railties/lib/rails/generators/testing/behaviour.rb#L90), which is then included in the `generator_spec` test case [here](https://github.com/stevehodgkiss/generator_spec/blob/4f321b217abf9d55b958c0846293ce2390a53f80/lib/generator_spec/test_case.rb#L14), which is then extended in the `rspec` "port" [here](https://github.com/stevehodgkiss/generator_spec/blob/4f321b217abf9d55b958c0846293ce2390a53f80/lib/generator_spec/generator_example_group.rb#L5), which is run on this file because of `type: :generator`.

Long story short, its a helper defined in the `railties` helpers