test(internal/tools/verifyreadmes): add unit tests for verifyReadme

[marcschaeferger]

This PR adds unit tests for the verifyreadmes tool.

The tests target the verifyReadme function in
internal/tools/verifyreadmes/main.go, which is responsible for ensuring
that each Go module directory contains a README.md.

Test coverage includes:

• propagation of incoming errors from filepath.Walk
• ignoring files that are not go.mod
• ignoring paths located in excluded directories
• successful validation when README.md exists alongside go.mod
• returning the expected error when README.md is missing

The missing README test validates the exact error message rather than
only asserting the presence of an error, addressing review feedback from
the previous attempt in #7026.

No changes were made to production code. This PR only introduces tests.
--> So no need to update the Changelog?

The following commands were run without any issue.

cd internal/tools/ && go test ./verifyreadmes/...
make precommit 

Closes #6604

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (7e13b02) to head (6a6219b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8066   +/-   ##
=====================================
  Coverage   81.6%   81.6%           
=====================================
  Files        304     304           
  Lines      23448   23448           
=====================================
+ Hits       19140   19141    +1     
+ Misses      3926    3925    -1     
  Partials     382     382           

see 2 files with indirect coverage changes

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