These tests are ok but could be modernised a bit

- use `safeAwait` instead of awaiting the latch directly
- move assertions into handler, avoiding the need for `responseHolder`
- would it be better to use an `ActionListenerResponseHandler` rather than a raw `TransportResponseHandler`?
server/src/test/java/org/elasticsearch/action/admin/cluster/configuration/TransportAddVotingConfigExclusionsActionTests.java
new AddVotingConfigExclusionsRequest(new String[] { "other1", "other2" }, Strings.EMPTY_ARRAY, TimeValue.timeValueSeconds(30)),
            expectSuccess(r -> {
                assertNotNull(r);
                countDownLatch.countDown();
            })
        );
        assertTrue(countDownLatch.await(30, TimeUnit.SECONDS));
        assertThat(
            clusterService.getClusterApplierService().state().getVotingConfigExclusions(),
            containsInAnyOrder(otherNode1Exclusion, otherNode2Exclusion)
        );
    }
    public void testExcludeAbsentNodesByNodeNames() throws InterruptedException {
        final CountDownLatch countDownLatch = new CountDownLatch(2);
        clusterStateObserver.waitForNextChange(new AdjustConfigurationForExclusions(countDownLatch));
        transportService.sendRequest(
            localNode,
            AddVotingConfigExclusionsAction.NAME,
            new AddVotingConfigExclusionsRequest("absent_node"),
            expectSuccess(e -> countDownLatch.countDown())
        );
        assertTrue(countDownLatch.await(30, TimeUnit.SECONDS));
        assertEquals(
            Set.of(new VotingConfigExclusion(VotingConfigExclusion.MISSING_VALUE_MARKER, "absent_node")),
            clusterService.getClusterApplierService().state().getVotingConfigExclusions()
        );
    }
    public void testExcludeExistingNodesByNodeNames() throws InterruptedException {
        final CountDownLatch countDownLatch = new CountDownLatch(2);
        clusterStateObserver.waitForNextChange(new AdjustConfigurationForExclusions(countDownLatch));