Fix cluster slot migration flaky test

tests/support/cluster_util.tcl

@@ -458,3 +458,13 @@ proc wait_for_slot_state {srv_idx pattern} {
         fail "incorrect slot state on R $srv_idx: expected $pattern; got [get_open_slots $srv_idx]"
     }
 }
+
+# Returns the test index of a node given its node ID
+proc get_node_index_by_id {node_id} {
+    for {set i 0} {$i < [llength $::servers]} {incr i} {
+        if {[R $i CLUSTER MYID] eq $node_id} {
+            return $i
+        }
+    }
+    return -1  ;# not found
+}

tests/unit/cluster/cli.tcl

@@ -295,6 +295,15 @@ test {Migrate the last slot away from a node using valkey-cli} {
         set owner_r [valkey $owner_host $owner_port 0 $::tls]
         set owner_id [$owner_r CLUSTER MYID]
 
+        # get the index of the owner node
+        set owner_index [get_node_index_by_id $owner_id]
+
+        wait_for_condition 1000 50 {
+            [cluster_get_node_by_id $owner_index $newnode_id] ne {}
+        } else {
+            fail "Owner node never learned the new node"
+        }
+
         # Move slot to new node using plain commands
         assert_equal OK [$newnode_r CLUSTER SETSLOT $slot IMPORTING $owner_id]
         assert_equal OK [$owner_r CLUSTER SETSLOT $slot MIGRATING $newnode_id]