tests: change main rule so that changes in nested tests won't run main

[maykathm]

This maps anything under tests/nested to nothing. With this change, in future PRs, if the PR's only change is a nested test, the non-nested systems will not run any tests.

Here's the change to the main.yaml rule file in action:

$ python3 tests/lib/external/snapd-testing-tools/utils/spread-filter -r tests/lib/spread/rules/main.yaml -p google::ubuntu-24.04-64 -c tests/nested/manual/some-test/task.yaml

$

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.28%. Comparing base (24a0034) to head (7602d96).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14889      +/-   ##
==========================================
+ Coverage   78.20%   78.28%   +0.08%     
==========================================
  Files        1151     1156       +5     
  Lines      151396   152624    +1228     
==========================================
+ Hits       118402   119489    +1087     
- Misses      25662    25763     +101     
- Partials     7332     7372      +40     

Flag	Coverage Δ
unittests	78.28% <ø> (+0.08%)	⬆️

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[olivercalder]

LGTM, thanks!