test_result: support templated URL for test results

The previous way of using portal.ERP5Site_absoluteUrl() +
test_result.getRelativeUrl() could not work with ERP5JS URLs.
To be compatible with this and also maybe erp5_web-style URLs if we want
to link to public tests results, we should have more flexibility in how
we generate URLs.
Introduce a "test result url template" property on gitlab connector, see
unit test for a renderjs ui compatible usage.