test(conf): add changelog_subject_only to expected settings

Followup to fix(changelog): the new default key broke
`tests/test_conf.py::TestReadCfg` because the expected `Settings`
literals pinned the full dict and didn''t know about the new field.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

tests/test_conf.py

@@ -100,6 +100,7 @@
     "changelog_incremental": False,
     "changelog_start_rev": None,
     "changelog_merge_prerelease": False,
+    "changelog_subject_only": False,
     "update_changelog_on_bump": False,
     "use_shortcuts": False,
     "major_version_zero": False,
@@ -140,6 +141,7 @@
     "changelog_incremental": False,
     "changelog_start_rev": None,
     "changelog_merge_prerelease": False,
+    "changelog_subject_only": False,
     "update_changelog_on_bump": False,
     "use_shortcuts": False,
     "major_version_zero": False,