RFR: 8309329: com/sun/jdi/DeferredStepTest.java fails with virtual threads due to not waiting for threads to exit

[Chris Plummer]

Virtual threads are always daemon threads, so tests that previously did not explicitly wait for test threads to exit sometimes fail with virtual threads due to the test exiting before the test threads have exited. A join() for each test thread is needed to fix this issue.

com/sun/jdi/DeferredStepTest.java is one such tests. I looked at the other com/sun/jdi failures listed in [JDK-8285422](https://bugs.openjdk.org/browse/JDK-8285422) and didn't see any others that might be failing for this same reason.

I tested locally with `JTREG_TEST_THREAD_FACTORY=Virtual`. I'll also run the appropriate mach5 tier that tests com/sun/jdi with virtual threads.

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Commit messages:
 - Use join() to make sure test threads have exited

Changes: https://git.openjdk.org/jdk/pull/14275/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14275&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309329
  Stats: 12 lines in 2 files changed: 9 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14275.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14275/head:pull/14275

PR: https://git.openjdk.org/jdk/pull/14275