Fix flakey test - org.apache.cassandra.index.sasi.SASIIndexTest testConcurrentMemtableReadsAndWrites

junit.framework.AssertionFailedError
  at org.apache.cassandra.index.sasi.utils.RangeIterator.<init>(RangeIterator.java:46)
  at org.apache.cassandra.index.sasi.memory.KeyRangeIterator.<init>(KeyRangeIterator.java:42)
  at org.apache.cassandra.index.sasi.memory.TrieMemIndex$ConcurrentTrie.search(TrieMemIndex.java:150)
  at org.apache.cassandra.index.sasi.memory.TrieMemIndex.search(TrieMemIndex.java:102)
  at org.apache.cassandra.index.sasi.memory.IndexMemtable.search(IndexMemtable.java:70)
  at org.apache.cassandra.index.sasi.conf.ColumnIndex.searchMemtable(ColumnIndex.java:138)
  at org.apache.cassandra.index.sasi.TermIterator.build(TermIterator.java:91)
  at org.apache.cassandra.index.sasi.plan.QueryController.getIndexes(QueryController.java:145)
  at org.apache.cassandra.index.sasi.plan.Operation$Builder.complete(Operation.java:434)
  at org.apache.cassandra.index.sasi.plan.QueryPlan.analyze(QueryPlan.java:57)
  at org.apache.cassandra.index.sasi.plan.QueryPlan.execute(QueryPlan.java:68)
  at org.apache.cassandra.index.sasi.SASIIndex.lambda$searcherFor$2(SASIIndex.java:301)
  at org.apache.cassandra.db.ReadCommand.executeLocally(ReadCommand.java:455)
  at org.apache.cassandra.index.sasi.SASIIndexTest.getIndexed(SASIIndexTest.java:2576)
  at org.apache.cassandra.index.sasi.SASIIndexTest.getPaged(SASIIndexTest.java:2537)
  at org.apache.cassandra.index.sasi.SASIIndexTest.testConcurrentMemtableReadsAndWrites(SASIIndexTest.java:1108)
  at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)