Flaky test: JanusGraphIndexTest.testSetIndexing

## Flaky Test

* **Test Name:** CQLSolrTest>JanusGraphIndexTest.testSetIndexing
* **Link:** https://github.com/JanusGraph/janusgraph/blob/8cb4bab52d5d88cd3f71c3ba845f18294e6abf1c/janusgraph-backend-testutils/src/main/java/org/janusgraph/graphdb/JanusGraphIndexTest.java#L2732
* **Branch:** master (PR: #3350)
* **Notes:**

### Stack Trace

```
Error: Tests run: 58, Failures: 0, Errors: 1, Skipped: 1, Time elapsed: 1,444.248 s <<< FAILURE! - in org.janusgraph.diskstorage.solr.CQLSolrTest
Error: testSetIndexing Time elapsed: 150.168 s <<< ERROR!
[...]
CQLSolrTest>JanusGraphIndexTest.testSetIndexing:2733->JanusGraphIndexTest.testIndexing:2756->JanusGraphIndexTest.testCollection:2814->JanusGraphIndexTest.clopen:235->JanusGraphBaseTest.clopen:210->JanusGraphIndexTest.open:228->JanusGraphBaseTest.open:137 » IllegalArgument
[INFO]
Error: Tests run: 379, Failures: 0, Errors: 1, Skipped: 133
```

Start with JanusGraphIndexTest.java around testSetIndexing at line 2732 and follow the stack trace through testIndexing, testCollection, and clopen. Reproduce CQLSolrTest>JanusGraphIndexTest.testSetIndexing and inspect the IllegalArgument error during open. Done means the test no longer fails intermittently in the reported CQLSolrTest run.

Written by the indexing model from the issue text.