Recognize ctest 'notrun' as skipped.

cmake 3.0 adds support for skipped tests, which are marked as not run:

  http://www.cmake.org/cmake/help/v3.0/prop_test/SKIP_RETURN_CODE.html#prop_test:SKIP_RETURN_CODE

PS: I haven't actually ran the test suite -- just ran the XSL manually
with xsltproc.