RFR: 8307091: Few test threw ConcurrentModificationException Intermittently

[Tejesh R]

Few test are throwing Concurrent Modification Exception Intermittently. Since the issue is intermittent and from the log and code review, It is found that the shared variable `fileCache` is used in comparison with the `newFileCache` where the comparison invokes the iterator method in AbstractList class. Since based on vector documentation it was evident that listIterator are fail-fast iterators and hence it would be better to make a separate local copy of vectors for comparing operations. 
The same is implemented and tested in CI system which shows green.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/13783/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13783&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307091
  Stats: 18 lines in 1 file changed: 8 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/13783.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13783/head:pull/13783

PR: https://git.openjdk.org/jdk/pull/13783