Update test_merge.py:TestMergeMulti.test_join_multi_levels
Base on the explanations in #16182, I think the `test_join_multi_levels` test below should be refactored. It looks correct because currently `dropna` is defaultly dropped from `index`. And I don't think `dropna` should be involved in this test.

what exactly are you trying to change here? if you want to add *another* test ok, but why are you changing this one? not sure what is the purpose here.