Added unit test for fdo#73872:

- Issue related to footer reference.
- Added unit test to check footer1.xml.rels
  is contains in doc after roundtrip.
- Also check Id = rId1 in footer1.xml.rels file.

Conflicts:
	chart2/qa/extras/chart2export.cxx
Reviewed on:
	https://gerrit.libreoffice.org/7851

Change-Id: I52fa8017abb7552c7752185873ff2bfb46689367