tests: workaround older boost::apply_visitor requiring a result_type member
tests: workaround older boost::apply_visitor requiring a result_type member

Older versions of boost::apply_visitor require a result_type member for the
visitor; supply it to make them happy.

Fixes #2312.

duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla