RFR: 8088343: Intermittent unit test failure in javafx.concurrent.ServiceLifecycleTest

[Andy Goryachev]

The code should not set the `Task.state` value to `CANCELLED` if the said task is already `SUCCEEDED` or `FAILED`.

This is a product bug.

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Commit messages:
 - 8088343

Changes: https://git.openjdk.org/jfx/pull/1769/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1769&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8088343
  Stats: 61 lines in 2 files changed: 37 ins; 14 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1769.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1769/head:pull/1769

PR: https://git.openjdk.org/jfx/pull/1769