RFR: 8309505: com/sun/jdi/MethodEntryExitEvents.java due to finding wrong main thread

[Chris Plummer]

The test fails because it tries to determine the main debuggee thread by allowing it run until the debuggee class is loaded (it waits for the ClassPrepareEvent). Normally this would be done on the main debuggee thread. However, when using virtual threads, the main thread has yet to spawn the virtual thread to run the test on. The test is fixed by instead just waiting until the debuggee main method is entered.

Tested by running the test locally with and without the virtual test thread wrapper, and also all of tier1 plus tier5 svc tests.

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Commit messages:
 - Fix virtual thread testing issue by waiting until debuggee main method is entered before gleaning the main thread.

Changes: https://git.openjdk.org/jdk/pull/14322/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14322&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309505
  Stats: 7 lines in 2 files changed: 1 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/14322.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14322/head:pull/14322

PR: https://git.openjdk.org/jdk/pull/14322