RFR: 8313290: Misleading exception message from STS.Subtask::get when task forked after shutdown

[Alan Bateman]

This is a small issue with StructuredTaskScope. If the owner attempts to fork a subtask and the task scope is shutdown then the implementation doesn't currently force the owner to join when it's the first fork in a new round of forking. The result is that broken code calling Subtask::get throws ISE with the wrong exception message.

The existing test for this API is updated to remove a duplicate test and also fix two tasks that didn't call join.

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Commit messages:
 - Merge
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/15076/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15076&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313290
  Stats: 23 lines in 2 files changed: 2 ins; 19 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15076.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15076/head:pull/15076

PR: https://git.openjdk.org/jdk/pull/15076