tests: automatically run nested tests for certain go changes

[maykathm]

This will automatically trigger nested tests if certain key areas of the go code are changed, while excluding changes to go tests. The change will be helpful in cases where a developer makes a change and should have added the "Run nested" label but forgot to do so.

As an example, any changes inside the kernel directory will automatically trigger nested tests:

$ tests/lib/external/snapd-testing-tools/utils/spread-filter -r tests/lib/spread/rules/nested.yaml -p "google:ubuntu-24.04-64" -c kernel/fde/fde.go 
google:ubuntu-24.04-64:tests/nested/

while changes to a key directory but to a unit test will not:

$ tests/lib/external/snapd-testing-tools/utils/spread-filter -r tests/lib/spread/rules/nested.yaml -p "google:ubuntu-24.04-64" -c overlord/devicestate/systems_test.go 

$

[github-actions]

Thu Feb 13 17:49:54 UTC 2025

Spread tests skipped

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@            Coverage Diff            @@
##             master   #15075   +/-   ##
=========================================
  Coverage          ?   78.06%           
=========================================
  Files             ?     1179           
  Lines             ?   157706           
  Branches          ?        0           
=========================================
  Hits              ?   123118           
  Misses            ?    26943           
  Partials          ?     7645           

Flag	Coverage Δ
unittests	78.06% <ø> (?)

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[Meulengracht]

Thank you for this change!

[alfonsosanchezbeato]

LGTM, thanks for this