testsuite: added mocked serial discovery for integration tests

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

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

	cli.InstallMockedSerialDiscovery(t)

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

An integration test that demonstrates the usage 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: 45 lines in your changes are missing coverage. Please review.

Comparison is base (1e51cdc) 62.82% compared to head (01e95f0) 62.77%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2376      +/-   ##
==========================================
- Coverage   62.82%   62.77%   -0.06%     
==========================================
  Files         204      205       +1     
  Lines       19311    19283      -28     
==========================================
- Hits        12132    12104      -28     
  Misses       6119     6119              
  Partials     1060     1060              

Flag	Coverage Δ
unit	62.77% <68.30%> (-0.06%)	⬇️

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

Files	Coverage Δ
commands/board/list.go	58.70% <100.00%> (ø)
commands/board/listall.go	91.13% <100.00%> (ø)
commands/compile/compile.go	66.78% <100.00%> (ø)
commands/core/download.go	57.14% <100.00%> (ø)
commands/core/install.go	71.42% <100.00%> (ø)
commands/core/list.go	86.95% <100.00%> (ø)
commands/core/search.go	84.21% <100.00%> (ø)
commands/core/uninstall.go	50.00% <100.00%> (ø)
commands/core/upgrade.go	80.64% <100.00%> (ø)
commands/debug/debug_info.go	66.86% <100.00%> (ø)
... and 19 more

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