No. The whole combination becomes retroflex because of the r. Phonetically what you get is ʈʂ.
First the rt becomes retroflex ʈ, then the s after also becomes retroflex, like ʂ.
I think the pronunciation here is pretty clear: http://sv.forvo.com/search/forts%C3%A4tta/
This happens in Standard Swedish and most dialects, but dialects that don't have the standard Swedish R sound (known as an alveolar trill: https://en.wikipedia.org/wiki/Alveolar_trill) will pronounce it differently.