Aaaaah, you want to run it over the test files themselves (I'm unsure how well this could work since I often like to make purposefully failing tests to assert that they fail, and this would also fail dialyzer in many cases).
Well in that case you could make a simple script to
elixirc the test exs files into a custom temporary output directory then run dialyzer over them. It would be fairly easy to make up a library to do that for a user if you want to make it? ^.^