Testing with subdomains

I am using this method of subdomains to access "shows" in my Rails project. In my controller tests, I set up a show with the subdomain "test":

class ShowsControllerTest < ActionController::TestCase
  def setup
    @show = Show.new(id: 1, name: 'My Show', user_id: 1, subdomain: 'test')
  end

  test "should do something"
    get show_path @show
  end
end

show_path is defined elsewhere:

def show_path(show)
  root_url(subdomain: show.subdomain)
end

Unfortunately, any test with get show_path @show causes this error (line returns added for ease of reading):

ActionController::UrlGenerationError:
  ActionController::UrlGenerationError:
    No route matches {:action=>"http://test.test.host/", :controller=>"shows"}

As far as I can tell, the issue is the hostname that the testing environment is using, and I can find a lot of articles about testing with subdomains, but they all related specifically to capybara and cucumber (I am using Ruby's default testing setup) and I cannot find sufficient parallels to fix the issue I am running into.