Fix cluster slot migration flaky test

tests/unit/cluster/cli.tcl

@@ -295,6 +295,22 @@ 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]
 
+        # Cluster check just verifies the config state is self-consistent,
+        # waiting for cluster_state to be okay is an independent check that all the
+        # nodes actually believe each other are healthy, prevent cluster down error.
+        wait_for_condition 1000 50 {
+            [catch {exec src/valkey-cli --cluster check 127.0.0.1:[srv 0 port]}] == 0 &&
+            [catch {exec src/valkey-cli --cluster check 127.0.0.1:[srv -1 port]}] == 0 &&
+            [catch {exec src/valkey-cli --cluster check 127.0.0.1:[srv -2 port]}] == 0 &&
+            [catch {exec src/valkey-cli --cluster check 127.0.0.1:[srv -3 port]}] == 0 &&
+            [CI 0 cluster_state] eq {ok} &&
+            [CI 1 cluster_state] eq {ok} &&
+            [CI 2 cluster_state] eq {ok} &&
+            [CI 3 cluster_state] eq {ok}
+        } else {
+            fail "Cluster doesn't stabilize"
+        }
+
         # 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]
@@ -530,4 +546,4 @@ start_multiple_servers 3 [list overrides $base_conf] {
     }
 }
 
-}
+}