Extended Reality (XR) headsets increasingly host familiar 2D user interfaces as windows floating in physical environments. However, their usability depends on environmental conditions that designers cannot fully control. Current design workflows often separate how a variant is designed from how it actually looks in the target environment. We present AdapTest, an in-situ authoring and testing tool that lets designers select interface elements, define when and how they change, compare the resulting variations, and test their activation without coding. Based on prior research and a formative study with 12 interface designers, the system targets three key environmental variables: distance, lighting, and occlusion. We evaluated AdapTest against a desktop design and in-headset preview workflow in a within-subjects study with 12 designers. Compared with the baseline, AdapTest led designers to account for a wider range of environmental factors, address more overlapping conditions, and repeatedly test and revise their designs in place; there were no significant differences in workload or usability despite longer sessions. Our work contributes an integrated, designer-centered approach to adapting, comparing, and validating environmentally responsive XR user interfaces.