Test viewer capability to handle a basic 'tref' element which points to a text string in an external file.
The test case consists of a single sub-test.
Run the test. No interaction required.
The box in the middle of the frame should contain green "Simple external referenced text.", which is obtained by a 'tref' element reference to a 'text' element in a 'defs' section of another file (text-extTref-BE-18-targ.svg).