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
transportService.sendRequest(
            localNode,
            AddVotingConfigExclusionsAction.NAME,
            new AddVotingConfigExclusionsRequest("other1", "other2"),
            expectSuccess(r -> {
                assertNotNull(r);
                countDownLatch.countDown();
            })
        );
        assertTrue(countDownLatch.await(30, TimeUnit.SECONDS));
        assertThat(
            clusterService.getClusterApplierService().state().getVotingConfigExclusions(),
            containsInAnyOrder(otherNode1Exclusion, otherNode2Exclusion)
        );
    }
    public void testSucceedsEvenIfAllExclusionsAlreadyAdded() throws InterruptedException {
        final ClusterState state = clusterService.state();
        final ClusterState.Builder builder = builder(state);
        builder.metadata(
            Metadata.builder(state.metadata())
                .coordinationMetadata(
                    CoordinationMetadata.builder(state.coordinationMetadata()).addVotingConfigExclusion(otherNode1Exclusion).build()
                )
        );
        setState(clusterService, builder);
        final CountDownLatch countDownLatch = new CountDownLatch(1);
        transportService.sendRequest(
            localNode,
            AddVotingConfigExclusionsAction.NAME,
            new AddVotingConfigExclusionsRequest("other1"),