tests: fixed querying new leader node in leadership transfer test

The test was using an api expecting the node ordinal to query for the
node that was elected as a leader. The query was based on the node id.
This lead to the validation being invalid.

Fixes: redpanda-data#19953

Signed-off-by: Michał Maślanka <michal@redpanda.com>

tests/rptest/tests/raft_availability_test.py

@@ -387,10 +387,12 @@ def test_leadership_transfer(self):
                                      leader_id=initial_leader_id)
         new_leader_id, _ = self._wait_for_leader(
             lambda l: l is not None and l != initial_leader_id)
+        new_leader_node = self.redpanda.get_node_by_id(new_leader_id)
+        assert new_leader_node is not None
         self.logger.info(
-            f"New leader is {new_leader_id} {self.redpanda.get_node(new_leader_id).account.hostname}"
+            f"New leader is {new_leader_id} {new_leader_node.account.hostname}"
         )
-        time.sleep(ELECTION_TIMEOUT)
+
         for [id, metric_check] in metric_checks.items():
             # the metric should be updated only on the node that was elected as a leader
             if id == new_leader_id: