tests: add spread suite for ruby plugin

[kyrofa]

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This PR resolves LP: #1783840 by moving the plugin and snapd integration tests for the Ruby plugin into a Spread suite.

[codecov-io]

Codecov Report

Merging #2212 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2212      +/-   ##
==========================================
- Coverage   91.21%   91.19%   -0.03%     
==========================================
  Files         208      208              
  Lines       13160    13160              
  Branches     1957     1957              
==========================================
- Hits        12004    12001       -3     
- Misses        788      789       +1     
- Partials      368      370       +2

Impacted Files	Coverage Δ
snapcraft/internal/pluginhandler/_plugin_loader.py	91.96% <0%> (-1.79%)	⬇️
snapcraft/internal/elf.py	82.84% <0%> (-0.33%)	⬇️

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