Flaky test: VirtualThreadPoolTest.testTrackingDump() — fails in 30% of CI builds

**Jetty version(s)**
12.1.x (12.1.8-SNAPSHOT through 12.1.9-SNAPSHOT)

**Jetty Environment**
core (jetty-util)

**Java version/vendor**
JDK 17, 21, 25

**OS type/version**
Linux (Jenkins CI)

**Description**

`VirtualThreadPoolTest.testTrackingDump()` is a frequently flaky test on the `jetty-12.1.x` CI branch. Over the last 60 builds (#1181–#1240), it failed as a flaky failure in **18 builds (30%)**.

**Typical failure:**

```
java.lang.AssertionError:
Expected: is <2>
but: was <0>
at VirtualThreadPoolTest.testTrackingDump(VirtualThreadPoolTest.java:133)
```

The test expects 2 tracked virtual threads in the dump output but finds 0, suggesting a timing issue where the virtual threads have not yet been registered or have already completed before the dump is taken.

**Frequency data (from Jenkins flaky test report):**

- 18 out of 60 builds affected (30% failure rate)
- Consistently present across the entire build history of the job

**Jenkins job:** https://jenkins.webtide.net/job/jetty.project/job/jetty-12.1.x/