test: perf: coroutinize run_worker()
test: perf: coroutinize run_worker()

Converts the executor::run_worker() method to a coroutine. This will
allow extending the function in further commits without having to
allocate continuations.

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