chore/add missing tests

[d-v-b]

This PR excludes some unreachable lines of code from test coverage, and adds tests for a few missing routines in zarr.api.asynchronous.

I put those missing tests in a new directory to ensure that the tests are found in a location that matches the layout of the source code (unlike our current api tests). Eventually we should move all of the tests in tests/test_api.py to tests/test_api/test_asynchronous.py or tests/test_api/test_synchronous.py.

I did not add a test for api.asynchronous.open_like because it's broken. see #3352 .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.64%. Comparing base (926a52f) to head (f6016db).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3353      +/-   ##
==========================================
+ Coverage   94.54%   94.64%   +0.10%     
==========================================
  Files          78       78              
  Lines        9423     9420       -3     
==========================================
+ Hits         8909     8916       +7     
+ Misses        514      504      -10     

Files with missing lines	Coverage Δ
src/zarr/abc/codec.py	98.50% <ø> (+4.22%)	⬆️

... and 1 file with indirect coverage changes

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