Extendes SwModelTestBase with mail merge functions and a declaration
of the DECLARE_MAILMERGE_TEST macro and uses it in a first test.
As most tests it's registered as a slow test run by 'make check'.

The broken MM used to drop the leading empty pages, which resulted
in a document with 4 pages and the two page bound draws merged
per single page.

Tests the MM result for page count (eight) and each draw anchor to
be on a different page.

Change-Id: Iab17f5844e68221d48cb89863323bcfe4c8ae0d2