Extend code-coverage to integration test

[cmaglie]

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What kind of change does this PR introduce?

It's a tentative implementation of the solution described here: https://go.dev/blog/integration-test-coverage

Starting from golang >=1.20 go build supports the -cover* flags, so, not only the tests, but even the actual executable may be instrumented to produce code-coverage output, BTW there are some non-trivial interesting aspects:

1. To produce the coverage output, the instrumented executable must be run with a GOCOVERDIR environment variable set to an existing directory path where the coverage data will be written. To accomplish this, I've added to the integration test the support for another env var INTEGRATION_GOCOVERDIR, the content of this variable will be passed to the arduino-cli run as GOCOVERDIR. We can not use GOCOVERDIR directly because it is overwritten by the go test ... command.
2. The coverage data is output in binary form and we must convert it to text to be digested by CodeCov. This is done with the command tool covdata textfmt -i=coverage_data -o coverage_integration.txt that picks the content of the coverage_data directory and transforms it in a coverage_integration.txt.
3. To upload the coverage data to CodeCov we must transfer the coverage output from each integration test job to the upload job. To accomplish this we exploit the upload_artifact capability to "add" files to an artifact, so it's just a matter of renaming the coverage_integration.txt file with a name that is unique among all integration test jobs. The current solution is: mv coverage_integration.txt coverage_integration_${{ matrix.operating-system }}_${{ matrix.tests }}.txt
4. Finally we merge all the coverage artifacts together to obtain a single coverage.txt file to send to CodeCov. This is required because the CodeCov upload action apparently does not support globs like coverate_integration*.txt (or at least I wasn't able to make it work). To do this I've used the very good gocovmerge tool.

What is the current behavior?

Code-coverage is calculated only for unit tests.

What is the new behavior?

Code-coverage is calculated on unit test and integration tests, reaching a whooping total of 61.78%!

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

No

Other information

Fix #1829

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +26.79 🎉

Comparison is base (245e84c) 35.05% compared to head (eacd81c) 61.85%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2103       +/-   ##
===========================================
+ Coverage   35.05%   61.85%   +26.79%     
===========================================
  Files         232      232               
  Lines       20480    19576      -904     
===========================================
+ Hits         7180    12108     +4928     
+ Misses      12455     6380     -6075     
- Partials      845     1088      +243     

Flag	Coverage Δ
unit	61.85% <ø> (+26.79%)	⬆️

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see 166 files with indirect coverage changes

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