tests: add lifecycle ordering tests

[kyrofa]

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• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?

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Also, generally refactor the lifecycle tests into their own package.

[sergiusens]

this rings a bell, I bet if we run blame we will see it was once a list 😛

[codecov-io]

Codecov Report

Merging #2163 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2163      +/-   ##
==========================================
+ Coverage   91.19%   91.21%   +0.02%     
==========================================
  Files         198      198              
  Lines       12471    12471              
  Branches     1846     1846              
==========================================
+ Hits        11373    11376       +3     
+ Misses        742      741       -1     
+ Partials      356      354       -2

Impacted Files	Coverage Δ
snapcraft/internal/lifecycle/_clean.py	85.08% <100%> (ø)	⬆️
snapcraft/internal/pluginhandler/__init__.py	92.42% <0%> (+0.32%)	⬆️
snapcraft/internal/lifecycle/_runner.py	92.85% <0%> (+0.59%)	⬆️

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