leak tests: Allow some more chances to find equilibrium

Author: jamadden

src/greenlet/tests/test_leaks.py

@@ -300,7 +300,7 @@ def test_issue251_issue252_explicit_reference_not_collectable(self):
             manually_collect_background=False,
             explicit_reference_to_switch=True)
 
-    UNTRACK_ATTEMPTS = 30
+    UNTRACK_ATTEMPTS = 100
 
     def _only_test_some_versions(self):
         # We're only looking for this problem specifically on 3.11,