refactor(testing): call super().__init__ in LatencyStore

[maxrjones]

Small code improvement to close out an old issue

Closes #2378

• 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/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (036ede7) to head (38021be).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4102   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          90       90           
  Lines       11981    11981           
=======================================
  Hits        11203    11203           
  Misses        778      778           

Files with missing lines	Coverage Δ
src/zarr/testing/store.py	99.43% <100.00%> (ø)

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