Have TP perform periodic checks of a file that you know will install. Write your code using an exist (or nonexist) syntax, then create a condition to handle your desired endstate. Seems like a like, but it's actually pretty simple.
Let me know if that works for you. I have tried to automate an install and there seems to be a bug in TestPartner with Installshield. If TestPartner is running, the install will never complete. Even if you pause your script and click the Finish button on the installer or you are running your install in silent mode, it will either never totally finish or will lock up the machine.