tubes test: fix stack corruption on test failure
tubes test: fix stack corruption on test failure

We can't use CPPUNIT_ASSERT() from a callback called by C code (such as
the FileSent callback), or we'll trash the stack if it fails.

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