Unittest generation of page number placeholders in table of contents

Conflicts:
	sw/inc/ToxTextGenerator.hxx
	sw/source/core/tox/ToxTextGenerator.cxx

Change-Id: I15c963b6e1a8823a1fdafd2c123d18ba3dc9f134
Reviewed-on: https://gerrit.libreoffice.org/9611
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>