Fix some deprecation warnings in tests.

[bdice]

This PR fixes a few separate warnings in pytests.

Examples:

../../../../../../opt/conda/envs/test/lib/python3.9/site-packages/hypothesis/_settings.py:467
  /opt/conda/envs/test/lib/python3.9/site-packages/hypothesis/_settings.py:467: HypothesisDeprecationWarning: `Healthcheck.all()` is deprecated; use `list(HealthCheck)` instead.
      The `hypothesis codemod` command-line tool can automatically refactor your code to fix this warning.
    note_deprecation(

../../../../../../opt/conda/envs/test/lib/python3.9/site-packages/cuml/internals/safe_imports.py:469
  /opt/conda/envs/test/lib/python3.9/site-packages/cuml/internals/safe_imports.py:469: DeprecationWarning: Please use `approx_fprime` from the `scipy.optimize` namespace, the `scipy.optimize.optimize` namespace is deprecated.
    return getattr(imported_module, symbol)

test_device_selection.py:192
  /__w/cuml/cuml/python/cuml/tests/test_device_selection.py:192: DeprecationWarning: `pytest_fixture_plus` and `fixture_plus` are deprecated. Please use the new alias `fixture`. See https://github.com/pytest-dev/pytest/issues/6475
    @pytest_fixture_plus(

[bdice]

Version 1.8.0 or higher contains a deprecation warning telling users to use the namespace scipy.optimize instead of scipy.optimize.optimize. It might be possible to use scipy.optimize in earlier versions but I know this is a safe lower bound. v1.8.0 was released Feb 5, 2022 so it's fine to require versions that new.

[bdice]

/merge