don't test self-deletion in stateful tests

[d-v-b]

This PR prevents the stateful group tests from deleting the root group

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[dcherian]

1. this will filter out foo/bar too no?

Suggested change

	members = tuple(filter(lambda v: "/" in v, sorted(self.all_groups)))
	members = tuple(filter(lambda v: v == "/", sorted(self.all_groups)))

2. Should this op be an error?

[d-v-b]

this will filter out foo/bar too no?

It will reject any member name that doesn't contain "/", so it will keep foo/bar

[dcherian]

I'm confused about how this is working. Can you add a comment please?

[d-v-b]

added in 724e3c9

[d-v-b]

let me know if this explanation needs more context, or if there's just a better way to get the same result -- I'm assuming it's a given that the name of the root group does not contain a "/" character.

[d-v-b]

closes #3244

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.62%. Comparing base (cea611b) to head (724e3c9).
⚠️ Report is 105 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3243      +/-   ##
==========================================
+ Coverage   92.25%   94.62%   +2.37%     
==========================================
  Files          78       78              
  Lines        8689     8690       +1     
==========================================
+ Hits         8016     8223     +207     
+ Misses        673      467     -206     

Files with missing lines	Coverage Δ
src/zarr/testing/stateful.py	98.20% <100.00%> (+59.88%)	⬆️

... and 2 files with indirect coverage changes

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