TST: Suppress warnings of drop_duplicates tests
Follow-up of #10236. Suppress unnecessary warnings during the test.

I don't think this is the correct fix. You shouldn't need to specify the `warnings.simplefilter('always', FutureWarning)` everywhere. This is done by `assert_produces_warning`. Or am I missing something?
 Correct, fixed to use `clear` argument of `assert_produces_warning`.
 @sinhrks do you know why the `clear` is needed in this case (otherwise we almost never use it)? 
 lgtm, merge when ready.
 Actually `clear` is not needed if we use `assert_produces_warning` in all cases. 

@jorisvandenbossche Pls merge if looks OK. 
 ah yes! Merging, thanks!
