RFR: 8364786: Test java/net/vthread/HttpALot.java intermittently fails - 24999 handled, expected 25000 [v2]

[Jaikiran Pai]

> Can I please get a review of this test-only change which proposes to fix an intermittent failure in that test? This addresses https://bugs.openjdk.org/browse/JDK-8364786.
> 
> As noted in that issue, the test fails because the main thread doesn't wait for the server side HttpHandler instances to return from the handle() method. There are 2 commits in this PR - one is general clean up and additional logging and the second one is the actual fix where we `close()` the server side `Executor` before asserting a counter state in the main thread. Closing the server side `Executor` waits till the HttpHandler instances return from their handle() method and thus prevents these intermittent test failures.
> 
> The reporter of the JBS issue has tested this proposed fix in their setup and the test no longer fails. I have also triggered a test repeat (and tier testing) for this change in our CI.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  Alan's review - move the test definition back to before the imports

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26686/files
  - new: https://git.openjdk.org/jdk/pull/26686/files/efdbbbb9..30044906

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26686&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26686&range=00-01

  Stats: 29 lines in 1 file changed: 15 ins; 14 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26686.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26686/head:pull/26686

PR: https://git.openjdk.org/jdk/pull/26686