Add macos testing to ci.yml

[lamBOOO]

This PR adds testing on macos, which was enabled with the v0.3.5-backport of PArrays.jl (see PartitionedArrays/PartitionedArrays.jl#212).

This fixes the macos related problems of #102.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.62%. Comparing base (96fb6f5) to head (51ee63e).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   85.96%   85.62%   -0.34%     
==========================================
  Files          10       10              
  Lines         969     1016      +47     
==========================================
+ Hits          833      870      +37     
- Misses        136      146      +10     

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