[CI] ThreadPoolMergeExecutorServiceDiskSpaceTests testBackloggedMergeTasksDoNotHoldUpBudget failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic-platform-support #8902 / almalinux-9_platform-support-unix
• elasticsearch-periodic #8827 / openjdk23_checkpart1_java-matrix
• elasticsearch-pull-request #76307 / part-1
• elasticsearch-intake #24166 / part1
• elasticsearch-periodic-platform-support #8794 / rhel-7_platform-support-unix

Reproduction Line:

./gradlew ":server:test" --tests "org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests.testBackloggedMergeTasksDoNotHoldUpBudget" -Dtests.seed=345024B57CF77B21 -Dtests.locale=jgo-CM -Dtests.timezone=America/Thule -Druntime.java=24

Applicable branches:
8.19

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: is <6575L>
     but: was <2136L>

Issue Reasons:

• [8.19] 5 failures in test testBackloggedMergeTasksDoNotHoldUpBudget (0.6% fail rate in 829 executions)
• [8.19] 2 failures in pipeline elasticsearch-periodic-platform-support (10.5% fail rate in 19 executions)

Note:
This issue was created using new test triage automation. Please report issues or feedback to es-delivery.

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)

[albertzaharovits]

https://gradle-enterprise.elastic.co/s/d3qwftaebm3fw
https://gradle-enterprise.elastic.co/s/zonrnnvzafqng
https://gradle-enterprise.elastic.co/s/nefqxy6mdovli

These failures are all due to inter-test interference, that's fixed by #130430 .

The others, I'm not so sure about, but the test is now unmuted, so let's wait and see.