Use addl_desc to determine spec type

We can determine spec_type by `desc`, but not `addl_desc`.

This change should be 100% backwards compatible. It will allow minitest-rails to register spec types by the additional spec. So folks can create tests using the spec DSL like so:

```ruby
class ActionDispatch::IntegrationTest
  register_spec_type(self) do |desc, addl_desc|
    addl_desc == :integration
  end
end

describe "Some User Action", :integration do
  it "does some things" do
    self.class.ancestors.must_include ActionDispatch::IntegrationTest
  end
end
```