test(core): add coverage over new parallel implementations

[imrn99]

this adds coverage over additions of #201

it also fixes a problem highlighted by the new tests; the two_unsew methods were not correctly dealing with vertex attributes

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.19331% with 42 lines in your changes missing coverage. Please review.

Project coverage is 81.73%. Comparing base (fdb2ed9) to head (df5b4eb).

Files with missing lines	Patch %	Lines
honeycomb-core/src/cmap/dim2/link_and_sew.rs	36.00%	32 Missing ⚠️
honeycomb-core/src/attributes/tests.rs	98.21%	7 Missing ⚠️
honeycomb-core/src/cmap/dim2/tests.rs	96.87%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   75.29%   81.73%   +6.43%     
==========================================
  Files          41       41              
  Lines        6376     6866     +490     
==========================================
+ Hits         4801     5612     +811     
+ Misses       1575     1254     -321     

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[imrn99]

the windows CI has decided it won't run 👍