CI: Fix Codecov intermittently failing on coverage report upload

The patch explicitly adds the use of the `with.token` option, as
confirmed by others to improve the behaviour even on public repos.

The token is not required for public repos, but it makes the workflow
less likely to randomly fail because of a limit in codecov CI
capabilities [1].

The secret token doesn't always save from false positives though,
because GitHub Actions are supposedly buggy when handling secrets [2-4].

[1] codecov/codecov-action#557 (comment)
[2] codecov/codecov-action#557 (comment)
[3] codecov/codecov-action#598 (comment)
[4] https://github.com/orgs/community/discussions/25701

.github/workflows/tests.yml

@@ -54,4 +54,18 @@ jobs:
       - name: Upload coverage results to Codecov
         uses: codecov/codecov-action@v3
         with:
+
+          # The token is not required for public repos, but it makes the workflow
+          # less likely to randomly fail because of a limit in codecov CI
+          # capabilities [1].
+          #
+          # The secret token doesn't always save from false positives though,
+          # because GitHub Actions are supposedly buggy when handling secrets [2-4].
+          #
+          # [1] https://github.com/codecov/codecov-action/issues/557#issuecomment-1216749652
+          # [2] https://github.com/codecov/codecov-action/issues/557#issuecomment-1223737955
+          # [3] https://github.com/codecov/codecov-action/issues/598#issuecomment-1223307820
+          # [4] https://github.com/orgs/community/discussions/25701
+          token: ${{ secrets.CODECOV_TOKEN }}
+
           fail_ci_if_error: true