TestSimlarity2.testNoFieldSkew() failure [LUCENE-6751]

ASF Jenkins found a failing seed for `TestSimilarity2.testNoFieldSkew()` that reproduces for me on OS X:

```
[junit4] Suite: org.apache.lucene.search.similarities.TestSimilarity2
[junit4] 2> NOTE: download the large Jenkins line-docs file by running 'ant get-jenkins-line-docs' in the lucene directory.
[junit4] 2> NOTE: reproduce with: ant test -Dtestcase=TestSimilarity2 -Dtests.method=testNoFieldSkew -Dtests.seed=216F1F3C38561BEF -Dtests.multiplier=2 -Dtests.nightly=true -Dtests.slow=true -Dtests.linedocsfile=/x1/jenkins/lucene-data/enwiki.random.lines.txt -Dtests.locale=es_ES -Dtests.timezone=America/Catamarca -Dtests.asserts=true -Dtests.file.encoding=UTF-8
[junit4] FAILURE 0.21s J0 | TestSimilarity2.testNoFieldSkew <<<
[junit4] > Throwable #1: java.lang.AssertionError: expected:<0.21697770059108734> but was:<0.0>
[junit4] > at __randomizedtesting.SeedInfo.seed([216F1F3C38561BEF:89FA6BFD1F67BF87]:0)
[junit4] > at org.apache.lucene.search.similarities.TestSimilarity2.testNoFieldSkew(TestSimilarity2.java:198)
[junit4] > at java.lang.Thread.run(Thread.java:745)
[junit4] 2> NOTE: leaving temporary files on disk at: /x1/jenkins/jenkins-slave/workspace/Lucene-Solr-NightlyTests-trunk/lucene/build/core/test/J0/temp/lucene.search.similarities.TestSimilarity2_216F1F3C38561BEF-001
[junit4] 2> NOTE: test params are: codec=DummyCompressingStoredFields(storedFieldsFormat=CompressingStoredFieldsFormat(compressionMode=DUMMY, chunkSize=16060, maxDocsPerChunk=1, blockSize=6), termVectorsFormat=CompressingTermVectorsFormat(compressionMode=DUMMY, chunkSize=16060, blockSize=6)), sim=DefaultSimilarity, locale=es_ES, timezone=America/Catamarca
[junit4] 2> NOTE: Linux 3.13.0-52-generic amd64/Oracle Corporation 1.8.0_45 (64-bit)/cpus=4,threads=1,free=192996984,total=288882688
[junit4] 2> NOTE: All tests run in this JVM: [TestNewestSegment, TestIndexWriterOnJRECrash, TestSizeBoundedForceMerge, TestLogMergePolicy, TestLockFactory, TestMaxPosition, TestNeverDelete, TestMultiTermQueryRewrites, TestToken, TestIndexInput, TestDocValues, TestSimpleExplanationsOfNonMatches, TestVirtualMethod, TestAllFilesHaveChecksumFooter, TestHighCompressionMode, TestFilterLeafReader, TestHugeRamFile, TestSearcherManager, TestIndexWriterCommit, TestTrackingDirectoryWrapper, Test4GBStoredFields, TestCachingCollector, TestIsCurrent, TestApproximationSearchEquivalence, Test2BPostings, TestRoaringDocIdSet, TestIndexWriterFromReader, TestNumericDocValuesUpdates, TestExitableDirectoryReader, TestTermScorer, TestDirectPacked, TestLazyProxSkipping, TestIndexWriterUnicode, TestSimilarity2]
[junit4] Completed [229/401] on J0 in 0.58s, 8 tests, 1 failure <<< FAILURES!
```

---
Migrated from [LUCENE-6751](https://issues.apache.org/jira/browse/LUCENE-6751) by Steven Rowe (@sarowe), updated May 09 2016
Linked issues:
- #7769

Start with org.apache.lucene.search.similarities.TestSimilarity2.testNoFieldSkew() at TestSimilarity2.java:198 and reproduce it using the supplied ant command, seed, and test parameters. Download the Jenkins line-docs file with ant get-jenkins-line-docs if needed. Done means the reported seed no longer fails and the expected similarity value is produced.

Written by the indexing model from the issue text.