tests: add spread suite for ament plugin

[kyrofa]

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This PR resolves LP: #1783827 by moving the snapd integration test for the Ament plugin into a Spread suite.

[codecov-io]

Codecov Report

Merging #2211 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2211      +/-   ##
=========================================
- Coverage   91.21%   91.2%   -0.02%     
=========================================
  Files         208     208              
  Lines       13160   13160              
  Branches     1957    1957              
=========================================
- Hits        12004   12002       -2     
- Misses        788     789       +1     
- Partials      368     369       +1

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

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