Change test to avoid stack overflow with MN threads

When using MN threads (such as running the test in a ractor), this test
failed because it was raising a SystemStackError: stack level too deep.

This is because the machine stack is smaller under MN threads than on
the native main thread.

Author: etiennebarrie

test/ruby/test_hash.rb

@@ -2138,7 +2138,9 @@ def hash_iter_recursion(h, level)
 
   def test_iterlevel_in_ivar_bug19589
     h = { a: nil }
-    hash_iter_recursion(h, 200)
+    # Recursion level should be over 127 to actually test iterlevel being set in an instance variable,
+    # but it should be under 131 not to overflow the stack under MN threads/ractors.
+    hash_iter_recursion(h, 130)
     assert true
   end