Generate fixture for #1312 if it is missing

[joshmoore]

This is a temporary fix for the larger issue of out-of-tree testing described in #1347, but this should allow a release of 2.14.1 which passes on conda.

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/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

Merging #1348 (aab38f6) into main (4e8b84b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1348   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines        14744     14752    +8     
=========================================
+ Hits         14744     14752    +8     

Impacted Files	Coverage Δ
zarr/tests/test_attrs.py	100.00% <100.00%> (ø)
zarr/util.py	100.00% <0.00%> (ø)
zarr/indexing.py	100.00% <0.00%> (ø)
zarr/tests/test_hierarchy.py	100.00% <0.00%> (ø)

[jakirkham]

Maybe this code provides an idea on how to approach fixtures?

zarr-python/zarr/tests/test_storage.py

Line 2326 in c375030

fixture = group(store=DirectoryStore('fixture'))