Add test for user-defined literal applied to integer

tests/core/DataTypesTest.cpp

@@ -53,6 +53,8 @@ int main( int /*argc*/, char** /*argv*/ )
    WALBERLA_CHECK_EQUAL( math::int_ld<  1 >::exp, 0 );
 
    WALBERLA_CHECK_IDENTICAL(1.23456_r, real_c(1.23456));
+   WALBERLA_CHECK_IDENTICAL(1_r, real_c(1));
+   WALBERLA_CHECK_IDENTICAL(-1_r, real_c(-1));
 
    return EXIT_SUCCESS;
 }