RFR: 8307486: ProcessTools.java should wait until vthread is completed before checking exceptions

[Leonid Mesnik]

Updated processtools to check exception after join().

Tested with running CI virtual thread tests.

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Commit messages:
 - 8307486: ProcessTools.java should wait until vthread is completed before checking exceptions

Changes: https://git.openjdk.org/jdk/pull/13873/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13873&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307486
  Stats: 4 lines in 1 file changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13873.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13873/head:pull/13873

PR: https://git.openjdk.org/jdk/pull/13873