Mute testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges (#129871)

Relates https://github.com/elastic/elasticsearch/issues/129761

Mute `testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges`
in `ThreadPoolMergeExecutorServiceDiskSpaceTests`. This is currently
failing 50% of times and affects backports to 8.19.

muted-tests.yml

@@ -458,3 +458,6 @@ tests:
   - class: org.elasticsearch.index.engine.ThreadPoolMergeSchedulerTests
     method: testSchedulerCloseWaitsForRunningMerge
     issue: https://github.com/elastic/elasticsearch/issues/125236
+  - class: org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests
+    method: testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges
+    issue: https://github.com/elastic/elasticsearch/issues/129761