TechTool Pro officially supports Apple's Fusion drive. All tests and tools can be used safely with one important caveat. 

Optimization (defragmentation) is not recommended for any disk that incorporates an SSD component. Optimization on Solid State drives doesn't speed up data access because all data is accessed at the same speed regardless of physical location. In addition, solid state drives utilize a feature called wear-leveling which uses all of the sectors of the drive evenly to extend the life of the drive. Optimizations work against this feature, wearing out the SSD sooner. When it comes to Fusion Drives specifically, optimizing may actually slow down the performance of your Mac if often used data is moved off the SSD portion of the drive during the optimization. So, the File and Volume Optimization tools will warn you against running them on your Fusion equipped Mac.