pybind/mgr: s/setup(self)/setup_method(self)/

pybind/mgr: s/setup(self)/setup_method(self)/

avoid pytest warnings like:

4: pg_autoscaler/tests/test_cal_final_pg_target.py::TestPgAutoscaler::test_even_pools_one_meta_three_bulk
4:   /home/kefu/dev/ceph/src/pybind/mgr/.tox/py3/lib/python3.10/site-packages/_pytest/fixtures.py:900: PytestRemovedIn8Warning: Support for nose tests is deprecated and will be removed in a future release.
4:   pg_autoscaler/tests/test_cal_final_pg_target.py::TestPgAutoscaler::test_even_pools_one_meta_three_bulk is using nose-specific method: `setup(self)`
4:   To remove this warning, rename it to `setup_method(self)`
4:   See docs: https://docs.pytest.org/en/stable/deprecations.html#support-for-tests-written-for-nose
4:     fixture_result = next(generator)

Signed-off-by: Kefu Chai <tchaikov@gmail.com>