Reduce allocations in diagonal tests

[jishnub]

Some of these allocations are unnecessary, and we may re-use pre-allocated arrays instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.81%. Comparing base (b265fea) to head (c171997).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1377   +/-   ##
=======================================
  Coverage   93.81%   93.81%           
=======================================
  Files          34       34           
  Lines       15717    15717           
=======================================
  Hits        14745    14745           
  Misses        972      972           

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