chore: improve Windows tests debug and small code cleanup

[lemeurherve]

This PR extracts tests debug improvement and small code cleanup from #388 and #295.

• We can now use -TestsDebug parameter or $env:TESTS_DEBUG to improve Windows tests debugging:
  • As it can be passed in the Jenkinsfile, it's easy now to activate it on a build replay instead of having to add a commit to switch debug on or off.
  • This parameter can take the following values:
    • empty/undefined: no additional debug (default value).
    • debug: output every test command and stderr on top of every test. (Equivalent of setting $quiet=$true in the test_helpers "Run-Program" function currently)
    • verbose: same as debug + output of stdout.
  • I tried using Powershell preference variables but didn't managed to get their values in the test_helpers functions without having to pass them around every call from ./build.ps1 to ./tests/sshAgent.ps1 to ./tests/test_helpers.ps1.
  • Using Run-Program in the beforeAll step of all tests involving SSH instead of calling docker directly so we can easily retrieve the commands used to reproduce them locally.
• Small code cleanup, mainly double quotes replacements by simple quotes, no functional change.
• Set build timeout from 30 to 60 minutes as builds take just about 30 minutes now.
• Added a section to the README explaining how to build and test Windows images locally.

Output:

With .\build.ps1 test -TestsDebug 'debug':

With .\build.ps1 test -TestsDebug 'verbose':

Testing done

Local tests + CI

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