leon: Update CORE_STRESS/list_sort since amount of CPU cycles increased.

With peak stack usage enabled the sorting method takes more CPU cycles and
with a list of 2000 entries it needs more than is provided by the test
suite.

leon-tests/CORE_STRESS/list_sort.exp

-0 1999
-1999 0
+0 1799
+1799 0

leon-tests/CORE_STRESS/list_sort.py

 # test large list sorting (should not stack overflow)
-l = list(range(2000))
+l = list(range(1800))
 l.sort()
 print(l[0], l[-1])
 l.sort(reverse=True)