TestThreadedForceMerge failure [LUCENE-3817]

leaks a file handle....

was running tests a lot to keep my computer busy while debugging something else.
```
[junit] Testsuite: org.apache.lucene.index.TestThreadedForceMerge
[junit] Testcase: testThreadedForceMerge(org.apache.lucene.index.TestThreadedForceMerge): Caused an ERROR
[junit] MockDirectoryWrapper: cannot close: there are still open files: {_58_1.skp=1, _58_1.tib=1, _58_1.tii=1, _58_0.frq=1, _58_0.tim=1, _58.tvf=1, _58.tvd=1, _58_1.doc=1, _58.tvx=1, _58.fdt=1, _58.fdx=1}
[junit] java.lang.RuntimeException: MockDirectoryWrapper: cannot close: there are still open files: {_58_1.skp=1, _58_1.tib=1, _58_1.tii=1, _58_0.frq=1, _58_0.tim=1, _58.tvf=1, _58.tvd=1, _58_1.doc=1, _58.tvx=1, _58.fdt=1, _58.fdx=1}
[junit] at org.apache.lucene.store.MockDirectoryWrapper.close(MockDirectoryWrapper.java:555)
[junit] at org.apache.lucene.index.TestThreadedForceMerge.testThreadedForceMerge(TestThreadedForceMerge.java:141)
[junit] at org.apache.lucene.util.LuceneTestCase$SubclassSetupTeardownRule$1.evaluate(LuceneTestCase.java:707)
[junit] at org.apache.lucene.util.LuceneTestCase$InternalSetupTeardownRule$1.evaluate(LuceneTestCase.java:606)
[junit] at org.apache.lucene.util.LuceneTestCase$TestResultInterceptorRule$1.evaluate(LuceneTestCase.java:511)
[junit] at org.apache.lucene.util.LuceneTestCase$RememberThreadRule$1.evaluate(LuceneTestCase.java:569)
[junit] at org.apache.lucene.util.LuceneTestCaseRunner.runChild(LuceneTestCaseRunner.java:165)
[junit] at org.apache.lucene.util.LuceneTestCaseRunner.runChild(LuceneTestCaseRunner.java:57)
[junit] Caused by: java.lang.RuntimeException: unclosed IndexInput: _58_1.doc
[junit] at org.apache.lucene.store.MockDirectoryWrapper.addFileHandle(MockDirectoryWrapper.java:479)
[junit] at org.apache.lucene.store.MockDirectoryWrapper.openInput(MockDirectoryWrapper.java:504)
[junit] at org.apache.lucene.codecs.mocksep.MockSingleIntIndexInput.(MockSingleIntIndexInput.java:41)
[junit] at org.apache.lucene.codecs.mocksep.MockSingleIntFactory.openInput(MockSingleIntFactory.java:32)
[junit] at org.apache.lucene.codecs.sep.SepPostingsReader.(SepPostingsReader.java:68)
[junit] at org.apache.lucene.codecs.mocksep.MockSepPostingsFormat.fieldsProducer(MockSepPostingsFormat.java:89)
[junit] at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsReader$1.visitOneFormat(PerFieldPostingsFormat.java:189)
[junit] at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$VisitPerFieldFile.(PerFieldPostingsFormat.java:280)
[junit] at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsReader$1.(PerFieldPostingsFormat.java:186)
[junit] at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsReader.(PerFieldPostingsFormat.java:186)
[junit] at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat.fieldsProducer(PerFieldPostingsFormat.java:256)
[junit] at org.apache.lucene.index.SegmentCoreReaders.(SegmentCoreReaders.java:108)
[junit] at org.apache.lucene.index.SegmentReader.(SegmentReader.java:51)
[junit] at org.apache.lucene.index.IndexWriter$ReadersAndLiveDocs.getMergeReader(IndexWriter.java:521)
[junit] at org.apache.lucene.index.IndexWriter.mergeMiddle(IndexWriter.java:3586)
[junit] at org.apache.lucene.index.IndexWriter.merge(IndexWriter.java:3256)
[junit] at org.apache.lucene.index.ConcurrentMergeScheduler.doMerge(ConcurrentMergeScheduler.java:382)
[junit] at org.apache.lucene.index.ConcurrentMergeScheduler$MergeThread.run(ConcurrentMergeScheduler.java:451)
[junit]
[junit]
[junit] Tests run: 1, Failures: 0, Errors: 1, Time elapsed: 0.704 sec
[junit]
[junit] ------------- Standard Error -----------------
[junit] NOTE: reproduce with: ant test -Dtestcase=TestThreadedForceMerge -Dtestmethod=testThreadedForceMerge -Dtests.seed=-36d98ee49f3f6205:-602830ebcb0e2f6d:9429978db44e64b -Dargs="-Dfile.encoding=UTF-8"
[junit] NOTE: test params are: codec=Lucene40: {id=PostingsFormat(name=MockSep), contents=PostingsFormat(name=NestedPulsing)}, sim=RandomSimilarityProvider(queryNorm=false,coord=true): {}, locale=ar_JO, timezone=Asia/Pontianak
[junit] NOTE: all tests run in this JVM:
[junit] [TestSearchForDuplicates, TestMockAnalyzer, TestToken, TestIntBlockCodec, TestImpersonation, TestBitVector, TestAtomicUpdate, TestBinaryTerms, TestCheckIndex, TestCrashCausesCorruptIndex, TestDirectoryReaderReopen, TestDocCount, TestDocValuesIndexing, TestForTooMuchCloning, TestIndexInput, TestIndexWriter, TestIndexWriterMerging, TestIndexWriterNRTIsCurrent, TestIndexWriterOnDiskFull, TestIndexWriterWithThreads, TestLazyProxSkipping, TestLongPostings, TestOmitTf, TestParallelAtomicReader, TestParallelCompositeReader, TestPayloadProcessorProvider, TestSameTokenSamePosition, TestSegmentTermDocs, TestSnapshotDeletionPolicy, TestSumDocFreq, TestTermVectorsWriter, TestThreadedForceMerge]
[junit] NOTE: Linux 3.0.0-14-generic amd64/Sun Microsystems Inc. 1.6.0_24 (64-bit)/cpus=8,threads=1,free=177836920,total=287309824
[junit] ------------- ---------------- ---------------
```

---
Migrated from [LUCENE-3817](https://issues.apache.org/jira/browse/LUCENE-3817) by Robert Muir (@rmuir)

Start by reproducing TestThreadedForceMerge.testThreadedForceMerge with the provided ant command and inspect TestThreadedForceMerge.java at line 141. Trace the unclosed inputs through MockDirectoryWrapper, MockSingleIntIndexInput, SepPostingsReader, and IndexWriter merge handling. Done means the test passes without open-file errors.

Written by the indexing model from the issue text.