RFR: 7903526: jtreg should handle exceptions thrown by virtual threads

[Leonid Mesnik]

Currently, jtreg executes a test in MainThreadGroup and checks uncaught exceptions for this group. So it handles all exceptions if the test doesn't create new thread groups.
However, the virtual threads don't belong to MainThreadGroup and jtreg silently ignores all exceptions thrown by virtual threads.

The testing shows that we have already some problems in jdk/jdk and probably in UR releases. There are 13 test failures in tier1 and several tens in the execution of tests that use secutiry-manager and run with a virtual thread.

It wonder if it makes sense to have this check conditional using some java property and start fixing tests after jtreg is release and smoothly switch execution to the new version.

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Commit messages:
 - jtreg should handle exceptions thrown by virtual threads

Changes: https://git.openjdk.org/jtreg/pull/172/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=172&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903526
  Stats: 38 lines in 2 files changed: 32 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jtreg/pull/172.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/172/head:pull/172

PR: https://git.openjdk.org/jtreg/pull/172