RFR: 8353718: Improve droppingMessage test in async UL tests [v3]

[Johan Sjölen]

> Hi,
> 
> The test for dropping messages have started to fail on aarch64 fast debug on Linux only. It's currently unclear why this occurs. To figure out why, I've extended the test to do emit more diagnostic information if it fails.
> 
> 1. Fail if the asynchronous mode isn't "drop" (the test makes no sense with stalling mode)
> 2. Print the entirety of the log file produced if no missing messages are found
> 3. If the thread running the test is unattached, then async UL will emit log messages in synchronous mode. Therefore, if the thread is unattached, the test now fails with an error message.
> 
> With these changes integrated, I hope to be able to find the actual culprit of this bug.
> 
> I have attempted to reproduce this bug on both mainline and with this patch, but I have been unable to do so.

Johan Sjölen has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Do not use BufferUpdater

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24411/files
  - new: https://git.openjdk.org/jdk/pull/24411/files/57c3877a..e4dc5eb6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24411&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24411&range=01-02

  Stats: 17 lines in 2 files changed: 0 ins; 12 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/24411.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24411/head:pull/24411

PR: https://git.openjdk.org/jdk/pull/24411