MINOR: Flaky test fixes (#21986)

Fixes a couple of flaky tests.

Reviewers: Chia-Ping Tsai <chia7712@gmail.com>, Apoorv Mittal
 <apoorvmittal10@gmail.com>

Author: AndrewJSchofield

tools/src/test/java/org/apache/kafka/tools/LeaderElectionCommandErrorTest.java

@@ -88,9 +88,11 @@ public void testMissingTopicPartitionSelection() {
 
     @Test
     public void testInvalidBroker() {
+        // Use RFC 5737 TEST-NET-1 (192.0.2.0/24) - a non-routable address reserved for
+        // documentation and testing. This address guarantees a connection timeout.
         Throwable e = assertThrows(AdminCommandFailedException.class, () -> LeaderElectionCommand.run(
             Duration.ofSeconds(1),
-            "--bootstrap-server", "example.com:1234",
+            "--bootstrap-server", "192.0.2.1:9092",
             "--election-type", "unclean",
             "--all-topic-partitions"
         ));