TestMergeSchedulerExternal failure [LUCENE-6750]

Policeman Jenkins found a failure on OS X that I can't reproduce on OS X 10.10.4 using Oracle Java 1.8.0_20, even after beasting 200 total suite iterations with the seed:

```
[junit4] Suite: org.apache.lucene.TestMergeSchedulerExternal
[junit4] 2> NOTE: reproduce with: ant test -Dtestcase=TestMergeSchedulerExternal -Dtests.method=testSubclassConcurrentMergeScheduler -Dtests.seed=3AF868F9E00E5EBA -Dtests.slow=true -Dtests.locale=ru -Dtests.timezone=Europe/London -Dtests.asserts=true -Dtests.file.encoding=ISO-8859-1
[junit4] FAILURE 0.37s J1 | TestMergeSchedulerExternal.testSubclassConcurrentMergeScheduler <<<
[junit4] > Throwable #1: java.lang.AssertionError
[junit4] > at __randomizedtesting.SeedInfo.seed([3AF868F9E00E5EBA:BD79D554E42E24BE]:0)
[junit4] > at org.apache.lucene.TestMergeSchedulerExternal.testSubclassConcurrentMergeScheduler(TestMergeSchedulerExternal.java:116)
[junit4] > at java.lang.Thread.run(Thread.java:745)
[junit4] 2> NOTE: test params are: codec=Asserting(Lucene53): {id=PostingsFormat(name=Memory doPackFST= true)}, docValues:{}, sim=DefaultSimilarity, locale=ru, timezone=Europe/London
[junit4] 2> NOTE: Mac OS X 10.8.5 x86_64/Oracle Corporation 1.8.0_51 (64-bit)/cpus=3,threads=1,free=16232544,total=54853632
[junit4] 2> NOTE: All tests run in this JVM: [TestDateSort, TestWildcardRandom, TestIndexWriterMergePolicy, TestPackedInts, TestSpansAdvanced, TestBooleanOr, TestParallelReaderEmptyIndex, TestFixedBitDocIdSet, TestIndexWriterDeleteByQuery, Test4GBStoredFields, TestMultiThreadTermVectors, TestIndexWriterConfig, TestToken, TestMergeSchedulerExternal]
[junit4] Completed [21/401] on J1 in 0.39s, 2 tests, 1 failure <<< FAILURES!
```

---
Migrated from [LUCENE-6750](https://issues.apache.org/jira/browse/LUCENE-6750) by Steven Rowe (@sarowe), updated Mar 07 2018

Start by running the reproduced ant test command for TestMergeSchedulerExternal.testSubclassConcurrentMergeScheduler with the supplied seed and parameters. Read TestMergeSchedulerExternal.java around line 116 and compare the failure behavior across the stated environments. Done means identifying the cause of the assertion failure and adding a reliable regression test or fix.

Written by the indexing model from the issue text.