Add a fuller integration test, and multiple minor changes to support it

[tigarmo]

This PR's main purpose is to add an integration test that exercises repo.install() covering the repository types that we currently support. In order to do this, I had to make multiple minor changes. The updates are applied over a handful of self-confined commits, which I hope will make the review easier. At merge time we can keep them as-is or squash 'em all, I don't have a strong preference either way.

CRAFT-1649 CRAFT-1682

[codecov[bot]]

Codecov Report

Merging #56 (1a7d636) into main (bc08456) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   83.96%   84.03%   +0.06%     
==========================================
  Files          13       13              
  Lines         786      789       +3     
  Branches      182      181       -1     
==========================================
+ Hits          660      663       +3     
  Misses         89       89              
  Partials       37       37              

Impacted Files	Coverage Δ
craft_archives/repo/apt_key_manager.py	98.19% <100.00%> (+0.05%)	:arrow_up:
craft_archives/repo/apt_preferences_manager.py	87.20% <100.00%> (ø)
craft_archives/repo/apt_sources_manager.py	93.75% <100.00%> (ø)

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