Fix reactive sentinel circuit breaker tests

[ruansheng8]

Describe what this PR does / why we need it

修复 Sentinel reactive 单元测试偶发不通过的情况 , 具体异常见:
https://github.com/alibaba/spring-cloud-alibaba/actions/runs/3845399772/jobs/6549501288

单元测试调用slow() 接口 , 每次调用延迟80ms

		@GetMapping("/slow")
		public Mono<String> slow() {
			return Mono.just("slow").delayElement(Duration.ofMillis(80));
		}

统计规则:

				factory.configure(
						builder -> builder.rules(Collections
								.singletonList(new DegradeRule("slow_mono").setCount(50)
										.setSlowRatioThreshold(0.7).setMinRequestAmount(5)
										.setStatIntervalMs(30000).setTimeWindow(5))),
						"slow_mono");

Unit test code:

	@Test
	public void test() throws Exception {
		StepVerifier.create(service.normal()).expectNext("normal").verifyComplete();
                // 开始调用slow接口 , 每次调用消耗80ms+程序自身执行损耗
		StepVerifier.create(service.slow()).expectNext("slow").verifyComplete();
		StepVerifier.create(service.slow()).expectNext("slow").verifyComplete();
		StepVerifier.create(service.slow()).expectNext("slow").verifyComplete();
		StepVerifier.create(service.slow()).expectNext("slow").verifyComplete();
		StepVerifier.create(service.slow()).expectNext("slow").verifyComplete();
                // 5次损耗: `5 * (80 + 程序自身损耗) > 400ms`
		// 这里循环5次调用 , 前4次耗时 `5 * (80 + 程序自身损耗) > 400ms`
		for (int i = 0; i < 5; i++) {
                        // 第5次调用时 , 上面所有耗时为 `400ms + 400ms + 4000ms + (程序损耗) > 4800ms`
                        // 当程序损耗 > 200ms 时 , 由于熔断时间窗口为5s , 所以重新计算 , 此时接口返回`slow` ,不符合预期的`fallback`
			StepVerifier.create(service.slow()).expectNext("fallback").verifyComplete();
			Thread.sleep(1000);
		}

修复方案:
减少循环时的等待时间 (1000ms -> 900ms) , 循环结束后等待500ms (补偿前面循环的等待时间 , 因为后面的单元测试需要在新的时间窗口中进行统计)

[steverao]

LGTM, thanks @ruansheng8 Please create a same PR to other main branch.