[skip-changelog] Migrate tests from test_cache.py to cache_test.go

[MatteoPologruto]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?

Infrastructure enhancement

What is the new behavior?

This PR is a followup to PR #1806
The integration test TestCacheClean, that was previously present in test_cache.py, has been rewritten in cache_test.go without changing its original purpose.

Does this PR introduce a breaking change, and is titled accordingly?

No

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[codecov]

Codecov Report

Merging #1839 (0b7eaf1) into master (c756472) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1839      +/-   ##
==========================================
+ Coverage   36.26%   36.34%   +0.07%     
==========================================
  Files         232      232              
  Lines       19496    19496              
==========================================
+ Hits         7071     7086      +15     
+ Misses      11594    11583      -11     
+ Partials      831      827       -4     

Flag	Coverage Δ
unit	36.34% <ø> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
arduino/cores/packagemanager/package_manager.go	63.02% <0.00%> (+0.84%)	⬆️
arduino/monitor/monitor.go	47.36% <0.00%> (+6.31%)	⬆️

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