RFR: 8370636: com/sun/jdi/TwoThreadsTest.java should wait for completion of all threads

[Leonid Mesnik]

Test 
com/sun/jdi/TwoThreadsTest.java 
start new threads using DebuggeeWrapper.newThread()
and exit immediately.

It is needed to jon() threads so test really works for virtual threads.

I looked on other threads that use DebuggeeWrapper.newThread(). Seems most of them already updated to join spawned threads.

Tested by running test on all platfrom with and wihtout virtual threads.

-------------

Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/27982/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27982&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370636
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27982.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27982/head:pull/27982

PR: https://git.openjdk.org/jdk/pull/27982