Avoid pandas Series.__getitem__ deprecation in tests

[jrbourbeau]

We're seeing

FAILED dask/dataframe/tests/test_dataframe.py::test_index_is_monotonic_dt64 - FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]`
FAILED dask/dataframe/io/tests/test_hdf.py::test_to_hdf_lock_delays - FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]`
FAILED dask/dataframe/tests/test_dataframe.py::test_apply - FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]`

in our upstream build due to pandas-dev/pandas#53201 (e.g. see this CI build)

This PR moves to iloc based selections to avoid the deprecation without changing the intent of any tests

cc @phofl

[phofl]

Lgtm, thx