tests: update relnotes test for actual output

I'm unclear why I didn't see this before but I consistently see it
now. In any event, it doesn't appear to be a regression, so let's just
bake it into the test.

Differential Revision: https://phab.mercurial-scm.org/D4463

tests/test-contrib-relnotes.t

@@ -3,6 +3,7 b''
 
   $ cd $TESTDIR/..
   $ python3 contrib/relnotes 4.4 --stoprev 4.5
+  changeset 3398603c5621: unexpected block in release notes directive feature
   New Features
   ============