gh-150114: Reduce memory usage of test_free_threading.test_iteration (gh-150115)

Reduce NUMITEMS from 100000 to 5000. Peak RSS for the full
test_free_threading suite drops from ~850 MB to ~175 MB.
(cherry picked from commit 61f1221)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Author: colesbury

Lib/test/test_free_threading/test_iteration.py

@@ -12,7 +12,7 @@
     NUMITEMS = 1000
     NUMTHREADS = 2
 else:
-    NUMITEMS = 100000
+    NUMITEMS = 5000
     NUMTHREADS = 5
 NUMMUTATORS = 2