With all respect, saying "JkDefrag will do actual work." is not the same as saying something like "Executing the screensaver 'Test' function does actually start defragging the disk."
With all respect, you are complaining about a function of the Operating System, not of JKDefrag.
Windows gives all screensavers the option to be "test run", for the user to determine if the screen they are seeing is an enjoyable customization option, or if they want to try a different screensaver from the list of choices given. This is the same function as the "preview" option that you get in the Screen Saver Settings dialog.
The normal expectation is that when you launch a screensaver, it will "go to work" right away; blanking the screen, displaying text or graphics in a changing pattern, enabling the system security options; and in the case of programs that display pictures from the disk or do other processing in the background, accessing the hard drive to retrieve or update information.
Placing a delay on that process runs counter intuitive to the reason for having a screen saver, since any user activity (and many background actions) will cause the screen saver to stop running and return the user to the desktop or to a login prompt if they so desire that security.
People are more likely to become suspicious of a "screen saver" program that doesn't appear to follow "normal" screen saver actions when it is launched.
One built in delay process that is already available for every screen saver is the countdown timer that the OS starts when it detects the system has gone idle, before attempting to start the screensaver. That can be set in the screen saver settings dialog.
Going back to your original question:
If you right-click on this .scr file, and select "Test", does that unintentionally actually start the real defragmentation process on the system disk?
The simple answer is: No, the starting of the defragmentation process is fully intentional
; the confusion is in how the term "test" is applied in this context. It is not JkDefrag running in a test mode that does no actions on the system, it is Windows running the screen saver program as a test to make sure it will actually start at the time the screen saver normally is called by the system.