Complex sentence: In the waterfall model, there is always a separate "testing phase" after a "build phase"; however, in agile development testing is usually done concurrently with, or at least in same iteration as, programming.
Split into simpler sentences: In the waterfall model, there is always a separate "testing phase" after a "build phase". However, in agile development testing is usually done concurrently with, or at least in same iteration as, programming.