ci: exclude tests/ from the coverage report on pull requests too (#6070)

The exclusion only worked on master. Coverage profiles name files by import
path; octocov shortens those to repo-relative paths using the checked-out
source, but coverage-on-pr.yml sparse-checks-out only .octocov.yml, so the
paths stay as github.com/navidrome/navidrome/tests/mock_*.go and 'tests/**'
never matched. '**/*_gen.go' matched either way, which is why only the 30
tests/ files leaked.

Every pull request since b77fb45 therefore reported ~-3.7% against master:
447 files on the base side, 477 on the pull request side (#6002, #6069).

.octocov.yml

@@ -8,8 +8,10 @@ coverage:
   paths:
     - coverage.out
   # Not code under test: tests/ holds the mocks and helpers, *_gen.go is generated.
+  # Both patterns need the '**/' prefix: the comment workflow has no source tree,
+  # so octocov cannot shorten the profile's import paths to repo-relative ones.
   exclude:
-    - 'tests/**'
+    - '**/tests/**'
     - '**/*_gen.go'
 codeToTestRatio:
   # Needs the pull request's own source, which the comment workflow must not