Fix ThreadPoolMergeExecutorServiceDiskSpaceTests testAbortingOrRunningMergeTaskHoldsUpBudget (#129979)

When there are multiple FS with the same available disk space but different total sizes, it's unpredictable (and irrelevant) which one the checker uses.

Fixes #129823

muted-tests.yml

@@ -551,9 +551,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.qa.single_node.GenerativeIT
   method: test
   issue: https://github.com/elastic/elasticsearch/issues/129819
-- class: org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests
-  method: testAbortingOrRunningMergeTaskHoldsUpBudget
-  issue: https://github.com/elastic/elasticsearch/issues/129823
 - class: org.elasticsearch.index.store.FsDirectoryFactoryTests
   method: testPreload
   issue: https://github.com/elastic/elasticsearch/issues/129852

server/src/test/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorServiceDiskSpaceTests.java

@@ -534,7 +534,7 @@ public class ThreadPoolMergeExecutorServiceDiskSpaceTests extends ESTestCase {
         aFileStore.totalSpace = randomLongBetween(1_000L, 10_000L);
         bFileStore.totalSpace = randomLongBetween(1_000L, 10_000L);
         aFileStore.usableSpace = randomLongBetween(900L, aFileStore.totalSpace);
-        bFileStore.usableSpace = randomLongBetween(900L, bFileStore.totalSpace);
+        bFileStore.usableSpace = randomValueOtherThan(aFileStore.usableSpace, () -> randomLongBetween(900L, bFileStore.totalSpace));
         boolean aHasMoreSpace = aFileStore.usableSpace > bFileStore.usableSpace;
         try (
             ThreadPoolMergeExecutorService threadPoolMergeExecutorService = ThreadPoolMergeExecutorService
@@ -613,7 +613,7 @@ public class ThreadPoolMergeExecutorServiceDiskSpaceTests extends ESTestCase {
         aFileStore.totalSpace = randomLongBetween(1_000L, 10_000L);
         bFileStore.totalSpace = randomLongBetween(1_000L, 10_000L);
         aFileStore.usableSpace = randomLongBetween(900L, aFileStore.totalSpace);
-        bFileStore.usableSpace = randomLongBetween(900L, bFileStore.totalSpace);
+        bFileStore.usableSpace = randomValueOtherThan(aFileStore.usableSpace, () -> randomLongBetween(900L, bFileStore.totalSpace));
         boolean aHasMoreSpace = aFileStore.usableSpace > bFileStore.usableSpace;
         try (
             ThreadPoolMergeExecutorService threadPoolMergeExecutorService = ThreadPoolMergeExecutorService
@@ -900,7 +900,7 @@ public class ThreadPoolMergeExecutorServiceDiskSpaceTests extends ESTestCase {
         long diskSpaceLimitBytes = randomLongBetween(10L, 100L);
         aFileStore.usableSpace = diskSpaceLimitBytes + randomLongBetween(1L, 100L);
         aFileStore.totalSpace = aFileStore.usableSpace + randomLongBetween(1L, 10L);
-        bFileStore.usableSpace = diskSpaceLimitBytes + randomLongBetween(1L, 100L);
+        bFileStore.usableSpace = randomValueOtherThan(aFileStore.usableSpace, () -> diskSpaceLimitBytes + randomLongBetween(1L, 100L));
         bFileStore.totalSpace = bFileStore.usableSpace + randomLongBetween(1L, 10L);
         boolean aHasMoreSpace = aFileStore.usableSpace > bFileStore.usableSpace;
         Settings.Builder settingsBuilder = Settings.builder().put(settings);
@@ -1001,7 +1001,10 @@ public class ThreadPoolMergeExecutorServiceDiskSpaceTests extends ESTestCase {
         bFileStore.totalSpace = randomLongBetween(300L, 1_000L);
         long grantedUsableSpaceBuffer = randomLongBetween(10L, 50L);
         aFileStore.usableSpace = randomLongBetween(200L, aFileStore.totalSpace - grantedUsableSpaceBuffer);
-        bFileStore.usableSpace = randomLongBetween(200L, bFileStore.totalSpace - grantedUsableSpaceBuffer);
+        bFileStore.usableSpace = randomValueOtherThan(
+            aFileStore.usableSpace,
+            () -> randomLongBetween(200L, bFileStore.totalSpace - grantedUsableSpaceBuffer)
+        );
         boolean aHasMoreSpace = aFileStore.usableSpace > bFileStore.usableSpace;
         Settings.Builder settingsBuilder = Settings.builder().put(settings);
         // change the watermark level, just for coverage and it's easier with the calculations