testsuite: added mocked serial monitor for integration tests

[cmaglie]

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What kind of change does this PR introduce?

Adds a mocked serial monitor to perform integration tests that require a working serial monitor.
The mocked monitor can be "implanted" with the command:

	cli.InstallMockedSerialMonitor(t)

This command will replace the original serial monitor with the mocked one. There is no way to restore the original monitor, so tests that do not require the mocking should be written separately.

The mocked serial monitor sends over the "emulated" serial port the name of the opened port and the initial port configuration.
If the monitor receives the string "QUIT" from the emulated serial port it quits and closes the port.
To actually allow the arduino-cli to send the QUIT command over the emulated port, I had to remove the IsTerminal check from the arduino-cli monitor command that prevented running arduino-cli monitor in a scripted environment (as the CI).

This change has the good side-effect of allowing the arduino-cli monitor command in a script to send data to the board.

An integration test that exercises the monitor command flags has been added as well.

What is the current behavior?

What is the new behavior?

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

Other information

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (710ecba) 62.77% compared to head (78e5d03) 63.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2379      +/-   ##
==========================================
+ Coverage   62.77%   63.59%   +0.82%     
==========================================
  Files         205      205              
  Lines       19283    19289       +6     
==========================================
+ Hits        12104    12266     +162     
+ Misses       6119     5931     -188     
- Partials     1060     1092      +32     

Flag	Coverage Δ
unit	63.59% <5.88%> (+0.82%)	⬆️

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

Files	Coverage Δ
internal/cli/monitor/monitor.go	62.77% <0.00%> (+39.87%)	⬆️
internal/cli/feedback/terminal.go	16.32% <8.33%> (+7.23%)	⬆️

... and 4 files with indirect coverage changes

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[alessio-perugini]

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