fdo#83751- Add test case for custom properties in pptx

Change-Id: I83fb0462c662e0da1f1c5c6dfec4bd8d94d97b87
Reviewed-on: https://gerrit.libreoffice.org/11420
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>