[jdk21u-dev] RFR: 8355569: Some nsk/jdi tests can glean the "main" thread by using the ClassPrepareEvent for the debuggee main class

[Goetz Lindenmaier]

I backport this to keep the tests up to date and make later backports clean.

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Depends on: https://git.openjdk.org/jdk21u-dev/pull/2404

Commit messages:
 - backport e2ae50d877b13b121912e2496af4b5209b315a05

Changes: https://git.openjdk.org/jdk21u-dev/pull/2406/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2406&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355569
  Stats: 166 lines in 39 files changed: 75 ins; 15 del; 76 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2406.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2406/head:pull/2406

PR: https://git.openjdk.org/jdk21u-dev/pull/2406