RFR: 8359827: Test runtime/Thread/ThreadCountLimit.java should run exclusively [v8]

[SendaoYan]

> Hi all,
> 
> The test runtime/Thread/ThreadCountLimit.java was observed fails when run with other tests. The test start subprocess with [shell prefix command](https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/runtime/Thread/ThreadCountLimit.java#L82) `ulimit -u 4096` which intend to limite the usage of thread number. But this will cause test fails when this test run with other tests. I create a demo to demonstrate that.
> 
> I start a java process which will create 5k threads, and then I can not start new java process with prefix `ulimit -u 4096` on the same machine.
> 
> <img width="1573" height="182" alt="image" src="https://github.com/user-attachments/assets/8dd53ade-cad7-4a2f-b5d9-d4b2d200b728" />
> 
> 
> [ManyThreads.java.txt](https://github.com/user-attachments/files/21324354/ManyThreads.java.txt)
> 
> So it's necessary to make this test run sperately to make this test success.
> Change has been verified locally, test-fix only, risk is low.

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

  Revert change to TEST.groups

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26401/files
  - new: https://git.openjdk.org/jdk/pull/26401/files/f49733d7..00cd0abb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26401&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26401&range=06-07

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26401.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26401/head:pull/26401

PR: https://git.openjdk.org/jdk/pull/26401