RFR: 8359131: JShell AbstractStopExecutionTest does not stop reliably

[Archie Cobbs]

The JShell regression test support class `AbstractStopExecutionTest.java` provides code that starts a JShell evaluation in one thread and then invokes `JShell.stop()` to interrupt that evaluation from another thread.

This class uses a `boolean` field to synchronize the action between threads. However, this field is not marked `volatile` and so the communication is unreliable. As a result, in rare cases, the test can fail due to a timeout.

The simple fix is to make the field `volatile`. This has been verified in testing to resolve the problem (see discussion in [JDK-8355323](https://bugs.openjdk.org/browse/JDK-8355323)).

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Commit messages:
 - Mark racy field volatile.

Changes: https://git.openjdk.org/jdk/pull/25730/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25730&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359131
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25730.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25730/head:pull/25730

PR: https://git.openjdk.org/jdk/pull/25730