RFR: Use ThreadGroup for passive connection

[moritz-bruder]

Passive connections use a separate thread to ensure interruptible IO. Since this is technically part of the test setup it should use a separate ThreadGroup.

Please let me know if anything else is needed or you want to see any changes. I was trying to follow your processes but found a lot of dead links.

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Commit messages:
 - Use root ThreadGroup as parent
 - Use null for ThreadGroup parameter
 - Use ThreadGroup for passive connection

Changes: https://git.openjdk.org/jtharness/pull/58/files
 Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=58&range=00
  Stats: 34 lines in 2 files changed: 30 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jtharness/pull/58.diff
  Fetch: git fetch https://git.openjdk.org/jtharness.git pull/58/head:pull/58

PR: https://git.openjdk.org/jtharness/pull/58