ducky/partition_balancer_test: fix full_node_test

Disable leader balancer after partition balancer has finished its work,
because leadership transfers will create new segments that can cause disk
usage to go over limit again even after we've verified that everything
is stable.

Fixes redpanda-data#9788

(cherry picked from commit f0e69dd)

tests/rptest/tests/partition_balancer_test.py

@@ -662,6 +662,12 @@ def is_ready_and_stable(s):
             nonlocal ready_appeared_at
             if s["status"] == "ready":
                 if ready_appeared_at is None:
+                    # Disable leader balancer after partition balancer has finished its work,
+                    # because leadership transfers will create new segments that can cause disk
+                    # usage to go over limit again even after we've verified that everything
+                    # is stable.
+                    self.redpanda.set_cluster_config(
+                        {"enable_leader_balancer": False})
                     ready_appeared_at = time.time()
                 else:
                     # ready status is stable for 11 seconds, should be enough for 3 ticks to pass