[chef] Using Minitest for "Other" Resources

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Sorry if this is a dumb question, but I'm not having much luck in finding an
answer.

I've been using minitests for a short while now in my recipes, so I have a
basic understanding of how it works.  What I'm trying to figure out is how to
test a resource that is not one of the pre-defined ones here:
https://github.com/calavera/minitest-chef-handler/blob/master/lib/minitest-chef-handler/resources.rb#L31

Specifically what I'm trying to do is something like: 
chef_gem("blah").must_be_installed

Whats the best/proper way to be able to that? I tried the above, and it of
course didn't work.  I was also trying to see if I could use the pre-defined
package resource and specify the provider, like you might do in a recipe,
without success.