Backport PR #2634 on branch 0.13.x (chore: use memory store in tests)

[flying-sheep]

Backports #2634

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.08%. Comparing base (5475617) to head (e40e23f).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/tests/helpers.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           0.13.x    #2639      +/-   ##
==========================================
- Coverage   87.96%   86.08%   -1.88%     
==========================================
  Files          49       49              
  Lines        7862     7879      +17     
==========================================
- Hits         6916     6783     -133     
- Misses        946     1096     +150     

Files with missing lines	Coverage Δ
src/anndata/experimental/merge.py	91.30% <100.00%> (+0.31%)	⬆️
src/anndata/tests/helpers.py	83.44% <93.33%> (-8.93%)	⬇️

... and 8 files with indirect coverage changes