Use non-consumable iterators for test fixtures

[d-v-b]

Pytest is emitting some deprecation warnings that are causing test failures, e.g. https://github.com/zarr-developers/zarr-python/actions/runs/27602093315/job/81605265762?pr=3987#step:7:61.

see https://docs.pytest.org/en/stable/deprecations.html#non-collection-iterables-in-pytest-mark-parametrize

we need to updated our tests to get ahead of this.

[d-v-b]

update: these failures are actually caused by a bug in pytest that will be fixed in the next release. I opened #4066 to ensure that we are not exposed to this kind of thing again.