Flaky Test: JanusGraphIndexTest.testDisableAndDiscardManuallyAndDropEnabledIndex

## Flaky Test

* **Test Name:** `JanusGraphIndexTest.testDisableAndDiscardManuallyAndDropEnabledIndex`
* **Link:** https://github.com/JanusGraph/janusgraph/blob/2c7007653a74ba01b938c7803a17cf17b1e3a9fe/janusgraph-backend-testutils/src/main/java/org/janusgraph/graphdb/JanusGraphIndexTest.java#L3771
* **Branch:** `master`, PR: #3650
* **Notes:** This failed for the job `[tests (es, -Pjava-11, -Pelasticsearch8, es8, 11)`

### Stack Trace

```
Error: Failures:
Error: BerkeleyElasticsearchTest>JanusGraphIndexTest.testDisableAndDiscardManuallyAndDropEnabledIndex:3788->JanusGraphIndexTest.registerIndex:3945 expected: but was:
```

cc @rngcntr as you've just added this test in #3362.

Start with janusgraph-backend-testutils/src/main/java/org/janusgraph/graphdb/JanusGraphIndexTest.java, especially testDisableAndDiscardManuallyAndDropEnabledIndex around line 3771 and registerIndex around line 3945. Run or inspect the failing Elasticsearch 8 Java 11 job and determine why the assertion is intermittent. Done means the test passes reliably in the affected job.

Written by the indexing model from the issue text.