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

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.

Author: nicoddemus

changelog/13480.bugfix.rst

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