tests: add spread suite for kbuild plugin

[kyrofa]

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This PR resolves LP: #1783835 by moving the plugin integration tests for the Kbuild plugin into a Spread suite.

[codecov-io]

Codecov Report

Merging #2203 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2203      +/-   ##
==========================================
+ Coverage   91.17%   91.19%   +0.02%     
==========================================
  Files         202      202              
  Lines       12987    12987              
  Branches     1921     1921              
==========================================
+ Hits        11841    11844       +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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