Compiler warnings from intel and nvcc

[PhilMiller]

Describe the bug

Commit 8315da, as built on the SNL machines

Intel:

/.../vt/tests/unit/lb/test_lb_data_comm.cc(135): warning #177: function "vt::tests::unit::comm::<unnamed>::ReduceMsg::serialize(SerializerT &) [with SerializerT=checkpoint::Serializer]" was declared but never referenced

    void serialize(SerializerT& s) {
         ^

nvcc the above, and additionally:

/.../vt/tests/unit/collection/test_collection_group.extended.cc(57): warning: variable "vt::tests::unit::handler_executed" was set but never used

[PhilMiller]

For Intel, the code is doing this, which should silence the warning:

#if defined(__INTEL_COMPILER)
#pragma warning(push)
#pragma warning(disable : 177)
#endif

[PhilMiller]

None of the (quasi-)documented approaches to quieting nvcc locally seem to work. So, I'm trying a workaround of just sticking (void)&ReduceMsg::serialize<checkpoint::Serializer>; in one of the test functions, to add a reference

[PhilMiller]

Follow-on to #1955 whose fix didn't seem to work