Fix test-cudf-python test directory.

[bdice]

This points test-cudf-python to the more specific tests directory so that it doesn't get confused by the python/benchmarks directory.

Error output:

________________________ ERROR collecting cudf/benchmarks/bench_cudf_io.py ________________________
ImportError while importing test module '~/rapids/cudf/python/cudf/cudf/benchmarks/bench_cudf_io.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../../compose/etc/conda/cuda_11.2/envs/rapids/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
cudf/benchmarks/bench_cudf_io.py:7: in <module>
    from conftest import option
E   ImportError: cannot import name 'option' from 'conftest' (~/rapids/cudf/python/cudf/cudf/tests/conftest.py)