RFR: 8304839: Move TestScaffold.main() to the separate class DebugeeWrapper

[Leonid Mesnik]

The code related to virtual threads support is moved into a separate class DebugeeWrapper.

The code of method main() remains the same. I don't mix code change with moving code between files.

Tesed by running 
make run-test JTREG_RETAIN=all  TEST=com/sun/jdi
make run-test JTREG_RETAIN=all  JTREG_TEST_THREAD_FACTORY=Virtual TEST=com/sun/jdi
locally and tier1 and hs-tier5 in Mach5 CI

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/15874/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15874&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304839
  Stats: 243 lines in 19 files changed: 125 ins; 86 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/15874.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15874/head:pull/15874

PR: https://git.openjdk.org/jdk/pull/15874