Hide the fact that the expression values are irrelevant for the unit test

clang was complaining that the values generated by the expressions passed to
CPPUNIT_ASSERT_THROW were never used. That is expected since they are
supposed to thrown an exception. Slightly reworking the expression
solved that problem.
Merge pull request #5710 from Dr15Jones/fixClangProblemsInDataFormatsFWLite

DataFormats/FWLite -- Hide the fact that the expression values are irrelevant for the unit test