drawingml export of sw textframe inset

This is triggered by testFdo66929 in CppunitTest_sw_ooxmlexport when DML
export of TextFrames is enabled in non-experimental mode.

Change-Id: I39481c55fc101aa9c61984cc5646911b4c207bc9
SwFiltersTest::filter: detect import filter on export test

This allows e.g. crash-testing the RTF export, even if the input source
is not RTF but ODT.

Change-Id: I3cf6afd1bc44bb475f1bd8da622d10dac0d3baa6