Replace manual spawning code in test case by kani::spawn library once it's merged

[fzaiser]

Replace the manual spawn code from #1658 with the library function, once #1659 is merged.

[rahulku]

@fzaiser - are you taking a look at this?

[fzaiser]

@rahulku Unfortunately, I'll be very busy for the next two to three months, so not any time soon.