CppunitTest_sw_htmlexport: make this really test the exported model

There were two problems here:

1) The original testsuite just served as a crash-test for the HTML
export filter, but the second testcase assumed it also reloads the
document model. Once this is fixed, we notice that:

2) There are some nasty 'mm100 <-> in' rounding errors, work this around
by using cms unconditionally.

(Exact problem was that CharTopBorderDistance was 457, but was expected
to be 450.)

Change-Id: Iaa1f98486767e4dfa81c2ca3e456bc39131977eb