GeneralEvents_OnStopTest(Sender) - differentiate manual stop from others

As I understand it, the "Sender" is an object that is used internally by TestComplete and is not something that can be manipulated by the script code.

However, here's a suggestion. Since you are going to by stopping the test manually, the assumption is that you are monitoring it and are watching it manually.  So... put a breakpoint in your OnStopTest handler code before the "cleanup" and, when you hit that breakpoint, just click the "stop" button in the IDE to stop all execution.