Added test TASK-COMPLETED-P/ITERATIONS to ensure that already

performed tasks are performed again if TASK-COMPLETE-P becomes NIL due
to a parameter in OPERATION-PLIST.
Expanded the TASK-COMPLETED-P method used in the TASK-COMPLETED-P/ITERATIONS test to compare against the current task value too.
Fixed bug in PERFORM, PERFORM-LEAF and TASK-COMPLETED-P which stopped the TASK-COMPLETED-P/ITERATIONS test from passing.