RFR: 7904105: intermittent test failure in CleanupDirTests.agentvm.conc

[Vladimir Petko]

jtharness does not synchronize calls to Observer. Synchronized keyword needs to be added when processing shared state.

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Commit messages:
 - fix: synchronize the test result callbacks

Changes: https://git.openjdk.org/jtreg/pull/297/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=297&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904105
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jtreg/pull/297.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/297/head:pull/297

PR: https://git.openjdk.org/jtreg/pull/297