tests: add spread suite for copy plugin

[kyrofa]

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This PR resolves LP: #1783831 by moving the integration tests for the copy plugin into a spread suite.

[codecov-io]

Codecov Report

Merging #2199 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2199      +/-   ##
==========================================
+ Coverage   91.13%   91.15%   +0.02%     
==========================================
  Files         201      201              
  Lines       12920    12920              
  Branches     1915     1915              
==========================================
+ Hits        11774    11777       +3     
+ Misses        777      776       -1     
+ Partials      369      367       -2

Impacted Files	Coverage Δ
snapcraft/internal/elf.py	83.17% <0%> (+0.32%)	⬆️
snapcraft/internal/pluginhandler/_plugin_loader.py	93.75% <0%> (+1.78%)	⬆️

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