Relax timeout in NodeConnectionsServiceTests (#42934)

Today we assert that the connection thread is blocked by the time the test gets
to the barrier, but in fact this is not a valid assertion. The following
`Thread.sleep()` will cause the test to fail reasonably often.

```diff
diff --git a/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java b/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java
index 193cde3180d..0e57211cec4 100644
--- a/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java
@@ -364,6 +364,7 @@ public class NodeConnectionsServiceTests extends ESTestCase {
             final CheckedRunnable<Exception> connectionBlock = nodeConnectionBlocks.get(node);
             if (connectionBlock != null) {
                 try {
+                    Thread.sleep(50);
                     connectionBlock.run();
                 } catch (Exception e) {
                     throw new AssertionError(e);
```

This change relaxes the test to allow some time for the connection thread to
hit the barrier.

Fixes #40170

server/src/test/java/org/elasticsearch/cluster/NodeConnectionsServiceTests.java

@@ -255,7 +255,7 @@ public class NodeConnectionsServiceTests extends ESTestCase {
 
             // once the connection is unblocked we successfully connect to it.
             nodeConnectionBlocks.clear();
-            connectionBarrier.await(0, TimeUnit.SECONDS);
+            connectionBarrier.await(10, TimeUnit.SECONDS);
             future3.actionGet();
             assertConnectedExactlyToNodes(nodes01);