test: fix box/bitset test failure

Reproduce file:

- [box/access.test.lua, null]
- [box/iterator.test.lua, null]
- [box/bitset.test.lua, null]

The issue happens, because box/bitset.lua:dump() uses iterate(), which
gets cleared by box/iterator test. Fix this by using utils.iterate()
instead.

test/box/lua/bitset.lua

@@ -36,7 +36,7 @@ function drop_space()
 end
 
 function dump(...)
-	return iterate('tweedledum', 'bitset', 1, 2, ...)
+	return utils.iterate('tweedledum', 'bitset', 1, 2, ...)
 end
 
 function test_insert_delete(n)