unit test XSheetAnnotations GetByIndex for #fdo80551

Change-Id: I89ebc3d5ac257f3c754a050caf3776959b81d8b3
Reviewed-on: https://gerrit.libreoffice.org/9933
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>