RFR: 8293591: Remove use of Thread.stop from jshell tests

[Jan Lahoda]

Two changes here:
- confirming the '/exit` command, so that the JShell instance is actually stopped
- removing the call to `Thread.stop`, which is intended to stop the test if something goes wrong unexpectedly. As we cannot use `Thread.stop` anymore, we'll rely on jtreg to stop the test.

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Commit messages:
 - 8293591: Remove use of Thread.stop from jshell tests

Changes: https://git.openjdk.org/jdk/pull/10271/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10271&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293591
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10271.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10271/head:pull/10271

PR: https://git.openjdk.org/jdk/pull/10271