obsolescence: add test for the "branch replacement" logic during push, case B6 Mercurial checks for the introduction of new heads on push. Evolution comes into play to detect if existing branches on the server are being replaced by some of the new one we push. This changeset adds test for the improved "branch replacement" logic introduce in an earlier commits. This tests initially lived in the evolve extensions. Since we now have the code handling this logic in core, it make sense to have the tests in core too. See inline documentation for details about the test case added in this changeset.