RFR: 8352568: Test gtest/AsyncLogGtest.java failed at droppingMessage_vm

[Johan Sjölen]

Hi,

The test for dropping messages have started to fail on aarch64 fast debug on Linux only when run as part of a test group. This seems to be a multi-threading issue with the `BufferUpdater`. When the `BufferUpdater` is removed, the issue disappears. This PR changes the code in a few other ways as well:

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.

If the test fails in the future, then these will give important diagnostic information regarding the state of the test.

This is a re-worked version of https://github.com/openjdk/jdk/pull/24411

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Commit messages:
 - Fix str
 - Do not use BufferUpdater
 - Improvements to async UL gtests

Changes: https://git.openjdk.org/jdk/pull/24508/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24508&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352568
  Stats: 73 lines in 4 files changed: 27 ins; 39 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/24508.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24508/head:pull/24508

PR: https://git.openjdk.org/jdk/pull/24508