Rename 13480.bugfix to 13480.contrib (#13554) (#13561)

Since this is related only to internal testing and does not affect end-users, it is more appropriate to announce it in the "Contributor" section of the changelog.

Follow up to #13481.

(cherry picked from commit cc580b5)

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

Author: patchback[bot]

changelog/13480.bugfix.rst

@@ -0,0 +1 @@
+Self-testing: fixed a few test failures when run with ``-Wdefault`` or a similar override.