Temporal correctness is crucial for the dependability of real-time control systems. A problem with testing such systems is the dependency on the execution orders of tasks. Mutation-based testing criteria have been proposed to determine which execution orders need to be exercised to verify that real-time systems are timely. For flexible control systems, timeliness in itself may only be relevant for a sub-set of tasks, whereas maintained control performance in the presence of worst-case jitter and disturbances is essential. This paper presents an extension to the co-simulator tool TrueTime, to support mutation-based testing of control performance and timeliness. Further, an approach for automatic generation of test cases using genetic algorithms is presented. A conclusion is that testing criteria for timeliness can be used to increase confidence in the dependability of flexible control systems.