Fix SearchableSnapshotDirectoryTests.testClearCache (#56277)

This test sometimes fails when prewarming is enabled because 
it's possible that some files are cached in background while the 
test tries to clear the cache. This commit disables prewarming 
for this test.

Author: tlrx

x-pack/plugin/searchable-snapshots/src/test/java/org/elasticsearch/index/store/SearchableSnapshotDirectoryTests.java

@@ -531,7 +531,9 @@ public void testClearCache() throws Exception {
                     shardId,
                     Settings.builder()
                         .put(SNAPSHOT_CACHE_ENABLED_SETTING.getKey(), true)
-                        .put(SNAPSHOT_CACHE_PREWARM_ENABLED_SETTING.getKey(), randomBoolean())
+                        // disable prewarming in this test to prevent files to be concurrently cached
+                        // while the cache is cleared out and while the test verifies it is empty
+                        .put(SNAPSHOT_CACHE_PREWARM_ENABLED_SETTING.getKey(), false)
                         .build(),
                     () -> 0L,
                     cacheService,