[jdk11u-dev] RFR: 8327137: Add test for ConcurrentModificationException in BasicDirectoryModel

[Andrew Lu]

I backport this for parity with 11.0.25-oracle.

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Commit messages:
 - Backport 9731b1c8b02d957985f4fb40acd93fb67747a9f0

Changes: https://git.openjdk.org/jdk11u-dev/pull/2775/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2775&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327137
  Stats: 278 lines in 1 file changed: 278 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2775.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2775/head:pull/2775

PR: https://git.openjdk.org/jdk11u-dev/pull/2775