Refactor test particle reading

[henry2004y]

• Construct struct with known pattern before loops for performance

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.75%. Comparing base (b8bc286) to head (fccde95).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   55.62%   55.75%   +0.13%     
==========================================
  Files          12       12              
  Lines        1688     1693       +5     
==========================================
+ Hits          939      944       +5     
  Misses        749      749              

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