tests that check private store methods

[d-v-b]

these tests check the existence / behavior of private store methods (_get_json, _get_bytes, etc) in zarr-python that we are considering removing in zarr-developers/zarr-python#3998.

To prevent yalls tests from breaking when we do remove these methods, those tests should be removed. I'm happy to open a PR to this effect!

[dcherian]

@d-v-b We would happily take that PR! These tests were blindly migrated wholesale in the early days just to ramp up coverage