Flaky test in SentinelDubboConsumerFilterTest (sentinel-apache-dubbo-adapter module)

## Issue Description

Type: *bug report*

### Describe what happened (or what feature you want)

Flaky test in `com.alibaba.csp.sentinel.adapter.dubbo.SentinelDubboConsumerFilterTest`:

```
[INFO] Running com.alibaba.csp.sentinel.adapter.dubbo.SentinelDubboConsumerFilterTest
[ERROR] Tests run: 6, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 2.271 s <<< FAILURE! - in com.alibaba.csp.sentinel.adapter.dubbo.SentinelDubboConsumerFilterTest
[ERROR] testDegradeAsync(com.alibaba.csp.sentinel.adapter.dubbo.SentinelDubboConsumerFilterTest) Time elapsed: 0.069 s <<< FAILURE!
org.junit.ComparisonFailure: expected:<[fallback]> but was:<[normal]>
at com.alibaba.csp.sentinel.adapter.dubbo.SentinelDubboConsumerFilterTest.testDegradeAsync(SentinelDubboConsumerFilterTest.java:115)

[INFO] Running com.alibaba.csp.sentinel.adapter.dubbo.DubboUtilsTest
[INFO] Tests run: 8, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.033 s - in com.alibaba.csp.sentinel.adapter.dubbo.DubboUtilsTest
[INFO]
[INFO] Results:
[INFO]
[ERROR] Failures:
[ERROR] SentinelDubboConsumerFilterTest.testDegradeAsync:115 expected:<[fallback]> but was:<[normal]>
```

Start by running SentinelDubboConsumerFilterTest and inspect testDegradeAsync at line 115, where the failure reports normal instead of fallback. Trace the asynchronous test setup and rerun it repeatedly to identify the source of the flakiness; done means the test reliably passes with the expected fallback result.

Written by the indexing model from the issue text.