Flaky Solr test

There appeared several times a failure of the `SolrIndexText#testTTL` in
- https://travis-ci.org/JanusGraph/janusgraph/jobs/373477718 on solr6
- https://travis-ci.org/JanusGraph/janusgraph/jobs/385071158 on solr5
- https://travis-ci.org/JanusGraph/janusgraph/jobs/371272709 on solr5
- https://travis-ci.org/JanusGraph/janusgraph/jobs/368598831 on solr5

With no seemingly related changes that can affect this test case. All of the failures look identically:
```
org.janusgraph.diskstorage.solr.SolrIndexTest
testTTL(org.janusgraph.diskstorage.solr.SolrIndexTest) Time elapsed: 6.187 sec <<< FAILURE!
java.lang.AssertionError: expected:<2> but was:<4>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:834)
at org.junit.Assert.assertEquals(Assert.java:645)
at org.junit.Assert.assertEquals(Assert.java:631)
at org.janusgraph.diskstorage.indexing.IndexProviderTest.testTTL(IndexProviderTest.java:772)
...
```

Start with IndexProviderTest.java around testTTL, then inspect SolrIndexTest#testTTL and the linked Travis CI failures for Solr 5 and Solr 6. Reproduce the intermittent expected-2/actual-4 result and trace the TTL behavior. Done means the test reliably passes while still checking the intended expiration behavior.

Written by the indexing model from the issue text.