test: raft: logical_timer: add abortable version of `sleep_until`
test: raft: logical_timer: add abortable version of `sleep_until`

`sleep_until(time_point tp, abort_source& as)` will sleep until `tp`, or
until `as.request_abort()` is called, whatever comes first.

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