Update relevant tests to use `sort_results` argument

#9130 adds a `sort_results` argument to `assert_eq`, which allows us to assert DataFrame equality _without_ sorting them (by default, `assert_eq` sorts the input DataFrames before checking equality). It'll be nice to identify+update the tests that would benefit from it. A first step could be to run the dataframe tests with `check_index=False` and `sort_results=False` as defaults.