writerfilter: move the RTF tests to slowcheck
writerfilter: move the RTF tests to slowcheck

These now take around 5sec here, which is slow enough to be annoying --
if you have to wait for it after every partial build.

Change-Id: I795fefcd79889aca7b00a5b3d408ee3488b13b03

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