[lworld] RFR: 8367589: [lworld] com/sun/jdi/valhalla tests fail with JTREG_TEST_THREAD_FACTORY=Virtual

[Alex Menkov]

Fixed issue with test class (TestScaffold) when it does not correctly handle "enablePreview" (does not pass "--enable-preview" to child debuggee process)

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Commit messages:
 - fix

Changes: https://git.openjdk.org/valhalla/pull/1620/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1620&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367589
  Stats: 6 lines in 2 files changed: 1 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1620.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1620/head:pull/1620

PR: https://git.openjdk.org/valhalla/pull/1620