RFR: 8303702: Provide ThreadFactory to create platform/virtual threads for com/sun/jdi tests

[Leonid Mesnik]

Provide a way to start debugee threads as platform or virtual for debugee in com/sun/jdi tests.

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Commit messages:
 - Added newThread method.

Changes: https://git.openjdk.org/jdk/pull/12894/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12894&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303702
  Stats: 51 lines in 4 files changed: 17 ins; 11 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/12894.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12894/head:pull/12894

PR: https://git.openjdk.org/jdk/pull/12894