To execute tests on a microcontroller, Tessy uses a third-party debugger (specific for the microcontroller in question) as interface to the test execution environment. Tessy controls the debugger during the tests. Therefore, the debugger must enable control by Tessy (e.g. by providing an API or script interface), and furthermore, Tessy must be adapted to the debugger in question.
It is of no relevance for Tessy whether the debugger operates as instruction-set simulator or as interface to a hardware debug system (JTAG/BDM/OCDS debugger, in-circuit emulator, or the like).