SwModelTestBase: support extracting node contents from a layout dump
SwModelTestBase: support extracting node contents from a layout dump

This way tests like "there is a textbox containing foo on the 3rd page"
are possible.

Also, make sure the XPath expression always matches a single node.

Change-Id: Iac82b389e1910db2257240a3764ec3c7ebaa5a02

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core