wmf/* tags no more trigger mediawiki-core-release

Found out that a bunch of tags for wmf commits have been added to
mediawiki/core such as: refs/tags/wmf/1.24wmf1

Prevent running mediawiki-core-release by using a branch: filter.
Add test to ensure we do it right.

Bug: T76085
Change-Id: Ib7089f0984274e39a293c4296dfdb6b7e4a8de04