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/TransportClearVotingConfigExclusionsActionTests.java
.coordinationMetadata(
                    CoordinationMetadata.builder()
                        .addVotingConfigExclusion(otherNode1Exclusion)
                        .addVotingConfigExclusion(otherNode2Exclusion)
                        .build()
                )
        );
        setState(clusterService, builder);
    }
    public void testClearsVotingConfigExclusions() throws InterruptedException {
        final CountDownLatch countDownLatch = new CountDownLatch(1);
        final SetOnce<ActionResponse.Empty> responseHolder = new SetOnce<>();
        final ClearVotingConfigExclusionsRequest clearVotingConfigExclusionsRequest = new ClearVotingConfigExclusionsRequest();
        clearVotingConfigExclusionsRequest.setWaitForRemoval(false);
        transportService.sendRequest(
            localNode,
            ClearVotingConfigExclusionsAction.NAME,
            clearVotingConfigExclusionsRequest,
            expectSuccess(r -> {
                responseHolder.set(r);
                countDownLatch.countDown();
            })
        );
        assertTrue(countDownLatch.await(30, TimeUnit.SECONDS));
        assertNotNull(responseHolder.get());
        assertThat(clusterService.getClusterApplierService().state().getVotingConfigExclusions(), empty());
    }
    public void testTimesOutIfWaitingForNodesThatAreNotRemoved() throws InterruptedException {
        final CountDownLatch countDownLatch = new CountDownLatch(1);
        final SetOnce<TransportException> responseHolder = new SetOnce<>();