Intel builds warn about unused code in test

[PhilMiller]

Seen in CDash for Intel 18 and 19:

[476/637] Building CXX object tests/CMakeFiles/lb_basic.dir/unit/lb/test_lb_data_comm.cc.o

/.../vt/tests/unit/lb/test_lb_data_comm.cc(131): warning #177: function "vt::tests::unit::comm::<unnamed>::ReduceMsg::serialize(SerializerT &) [with SerializerT=checkpoint::Serializer]" was declared but never referenced

    void serialize(SerializerT& s) {
         ^

[PhilMiller]

The message class is SerializeRequired, and that's probably appropriate for things used in reductions. So, let's suppress the warning

[PhilMiller]

https://github.com/kokkos/kokkos/pull/5433/files demonstrates how