Allow AlreadyClosedException to be a possible outcome of injecting fake exceptions in TestIndexFileDeleter

[dweiss]

We get occasional exceptions in tests like this one:
https://jenkins.thetaphi.de/job/Lucene-nightly-main-Linux/1070/console

org.apache.lucene.index.TestIndexFileDeleter > test suite's output saved to /home/jenkins/workspace/Lucene-nightly-main-Linux/lucene/core/build/test-results/test/outputs/OUTPUT-org.apache.lucene.index.TestIndexFileDeleter.txt, copied below:
   >     org.apache.lucene.store.AlreadyClosedException: ReaderPool is already closed
   >         at __randomizedtesting.SeedInfo.seed([807AD80E9847078:E19ADAB29F4D9785]:0)
   >         at app//org.apache.lucene.index.ReaderPool.get(ReaderPool.java:397)
   >         at app//org.apache.lucene.index.IndexWriter.writeReaderPool(IndexWriter.java:4000)
   >         at app//org.apache.lucene.index.IndexWriter.getReader(IndexWriter.java:607)
   >         at app//org.apache.lucene.index.IndexWriter$4.getReader(IndexWriter.java:6588)
   >         at app//org.apache.lucene.tests.index.RandomIndexWriter.getReader(RandomIndexWriter.java:502)
   >         at app//org.apache.lucene.tests.index.RandomIndexWriter.getReader(RandomIndexWriter.java:428)
   >         at app//org.apache.lucene.index.TestIndexFileDeleter.testExcInDecRef(TestIndexFileDeleter.java:481)

I (as well as claude...) think it is possible that the injected runtime exception may happen inside the concurrent merge scheduler's merge thread, causing the IW and the reader pool to be shut down. Then the test thread gets AlreadyClosedException on w.getReader().

I think allowing AlreadyClosedException is fine here.