threads: bugfix - do not call thread_current() from model-checker

thread_current() was designed for use in the user context. It is not
guaranteed to provide a reliable result in the model-checker context,
since we may perform context switches as needed, such that the "last
executed user thread" may not be the thread that we are checking at the
time.

This change is made to clear up future changes that will modify the
scheduling patterns.