Ensure at least 1 seed configured in remote test (elastic#49389)

This fixes elastic#49384. Currently when we select a random subset of seed
nodes from a list, it is possible for 0 seeds to be selected. This test
depends on at least 1 seed being selected.

server/src/test/java/org/elasticsearch/transport/RemoteClusterConnectionTests.java

@@ -490,8 +490,8 @@ public void testConnectedNodesConcurrentAccess() throws IOException, Interrupted
                 discoverableTransports.add(transportService);
             }
 
-            List<String> seedNodes = new CopyOnWriteArrayList<>(randomSubsetOf(discoverableNodes.stream()
-                .map(d -> d.getAddress().toString()).collect(Collectors.toList())));
+            List<String> seedNodes = new CopyOnWriteArrayList<>(randomSubsetOf(randomIntBetween(1, discoverableNodes.size()),
+                discoverableNodes.stream().map(d -> d.getAddress().toString()).collect(Collectors.toList())));
             Collections.shuffle(seedNodes, random());
 
             try (MockTransportService service = MockTransportService.createNewService(Settings.EMPTY, Version.CURRENT, threadPool, null)) {