tests: function-ize model.jl

[IainNZ]

Tested locally by running the file and running runtests.jl, seems fine?

Only marginally faster, order of a second on a ~40 second base.

[codecov]

Codecov Report

Merging #2303 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2303      +/-   ##
==========================================
- Coverage   91.39%   91.38%   -0.01%     
==========================================
  Files          42       42              
  Lines        4251     4250       -1     
==========================================
- Hits         3885     3884       -1     
  Misses        366      366              

Impacted Files	Coverage Δ
src/copy.jl	87.50% <0.00%> (-0.50%)	⬇️

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