[skip-changelog] Reuse already installed platforms to speed up integration tests

[cmaglie]

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• The PR follows
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• 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?

A lot of tests (especially the compile ones) are doing something like:

• core install arduino:avr (slow)
• perform the test (very quick)

What is the current behavior?

There are a lot of tests that take most part of the time just installing the same platform again and again.

What is the new behavior?

All the tests that require the same platform installed as a prerequisite are grouped together in a bigger test that runs each of them as a sub-test.
This change requires a bit more attention on the cleanup of the sub-tests since the leftovers of one test can be retrieved in a later test.

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

No

[codecov]

Codecov Report

Base: 36.65% // Head: 36.84% // Increases project coverage by +0.19% 🎉

Coverage data is based on head (c44c00c) compared to base (2480d93).
Patch coverage: 95.45% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1913      +/-   ##
==========================================
+ Coverage   36.65%   36.84%   +0.19%     
==========================================
  Files         231      235       +4     
  Lines       19683    19763      +80     
==========================================
+ Hits         7215     7282      +67     
- Misses      11640    11650      +10     
- Partials      828      831       +3     

Flag	Coverage Δ
unit	36.84% <95.45%> (+0.19%)	⬆️

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

Impacted Files	Coverage Δ
internal/integrationtest/shared_directory.go	82.60% <82.60%> (ø)
internal/integrationtest/arduino-cli.go	85.91% <100.00%> (+0.25%)	⬆️
internal/integrationtest/environment.go	100.00% <100.00%> (ø)
internal/integrationtest/http_server.go	100.00% <100.00%> (ø)
internal/integrationtest/subtests.go	100.00% <100.00%> (ø)
arduino/monitor/monitor.go	41.05% <0.00%> (-4.74%)	⬇️

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