RFR: 8288214: serviceability/jvmti/vthread/VThreadNotifyFramePopTest/VThreadNotifyFramePopTest.java test failed

[Alan Bateman]

This test connects to http://openjdk.java.net/ so it's not reliable if the host name can't be resolved or a HTTP connection cannot be established. I've changed the test to use a local HTTP server so the original test works as before, it's just a local rather than remote HTTP connection.

I did a few cleanups to the test while I was there:
- changed the test to use `@enablePreview`.
- renamed "VThread" in the comment to "virtual thread".
- changed the test to use Executors.newVirtualThreadPerTaskExecutor to avoid needing to specify a ThreadFactory.
- add a call Future::get so that if the virtual thread terminates with an exception then the test will fail quickly, avoids needing to calling into the native/agent code to check if it passed.

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Commit messages:
 - Merge
 - Initial commit

Changes: https://git.openjdk.org/jdk19/pull/6/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk19&pr=6&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288214
  Stats: 57 lines in 2 files changed: 38 ins; 4 del; 15 mod
  Patch: https://git.openjdk.org/jdk19/pull/6.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/6/head:pull/6

PR: https://git.openjdk.org/jdk19/pull/6