Flaky cache expiration test: ExpirationCacheTest.testExpiration

## Flaky Test

I saw the test fail already for two Dependabot PRs (both targeted on master) but it worked when I ran the test locally and also when I restarted the GH action.

* **Test Name:** ExpirationCacheTest.testExpiration
* **Link:** https://github.com/JanusGraph/janusgraph/blob/a60e92815b3a51a4cc020a7c7cb2bce9f414887a/janusgraph-test/src/test/java/org/janusgraph/diskstorage/cache/ExpirationCacheTest.java#L57
* **Branch:** master
* **Notes:** I'll create PR to mark it as flaky.

### Stack Trace

```
Error: Failures:
Error: ExpirationCacheTest.testExpiration:58->testExpiration:86->verifyResults:151 expected: <6> but was: <5>
```

Start with janusgraph-test/src/test/java/org/janusgraph/diskstorage/cache/ExpirationCacheTest.java, focusing on ExpirationCacheTest.testExpiration and the reported failure at lines 58–86. Run the test to confirm its intermittent result, then mark the test as flaky and verify that the test configuration recognizes the change.

Written by the indexing model from the issue text.