The previous implementation will generate URL like:
erp5/test_page_module/core/TestRunner.html?
Which missed the test created in portal_tests, the correct one should be:
erp5/test_page_module/portal_tests/user-Howto.Create.Persons/core/TestRunner.html?
This code was inspired by https://lab.nexedi.com/nexedi/erp5/blob/master/product/ERP5Type/tests/ERP5TypeFunctionalTestCase.py#L86