gate cargo test/example targets behind zarr feature

[elc45]

cargo test fails to compile crates with optional features off cause some tests are unconditionally trying to import Zarr. same with example targets

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.23%. Comparing base (394c629) to head (56c389d).
⚠️ Report is 116 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (394c629) and HEAD (56c389d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (394c629)	HEAD (56c389d)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #61       +/-   ##
===========================================
- Coverage   83.77%   61.23%   -22.54%     
===========================================
  Files           8       31       +23     
  Lines        1923     8008     +6085     
===========================================
+ Hits         1611     4904     +3293     
- Misses        312     3104     +2792     

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[aseyboldt]

Thanks, that's much cleaner!