Run slow tests in ci based on test-slow label

[maxrjones]

Addresses #735 (comment)

• Closes #xxxx
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.55%. Comparing base (51c9eed) to head (91d008c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
+ Coverage   87.45%   87.55%   +0.09%     
==========================================
  Files          34       35       +1     
  Lines        1818     1848      +30     
==========================================
+ Hits         1590     1618      +28     
- Misses        228      230       +2     

see 2 files with indirect coverage changes

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[maxrjones]

See zarr-developers/zarr-python#3264 (comment) for an explanation of the failing upstream tests