Merge: Introduce `test_frontend`

Merge: Introduce `test_frontend`

Sometimes we need to test things related to a model and it's not easy to do this with NitUnit since creating
a model by hand is tedious.

With the `TestModel` abstract suite it's easier:

```nit
module my_test is test

import test_frontend

class MyTest
   super TestModel
   test

   redef var test_src = "path/to/files"

   fun my_test is test do
       assert test_model.mmodules.length == 1
   end
end
```

Pull-Request: #2769
Reviewed-by: Jean Privat <jean@pryen.org>