test: raft: randomized_nemesis_test put `variant` and `monostate` `ostream` `operator<<`s into `std` namespace
test: raft: randomized_nemesis_test put `variant` and `monostate` `ostream` `operator<<`s into `std` namespace

As a preparation for the following commits.
Otherwise the definitions wouldn't be found during argument-dependent lookup
(I don't understand why it worked before but won't after the next commit).

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla