Fix Background Merge Breaking Snapshot Restore Test (#63579) (#64128)

If we run into a background merge between creating the snapshot and closing the index
then with compound files we could be in a situation where we get zero file reuse
on restore.
Force merging before the snapshot gives us a single segment that won't change down the line
so the restore always sees file reuse from the close index.

Closes #63476

Author: original-brownbear

server/src/internalClusterTest/java/org/elasticsearch/snapshots/DedicatedClusterSnapshotRestoreIT.java

@@ -43,7 +43,6 @@
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
 import org.elasticsearch.cluster.metadata.Metadata;
 import org.elasticsearch.cluster.node.DiscoveryNode;
-import org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.CheckedFunction;
 import org.elasticsearch.common.ParseField;
@@ -560,11 +559,8 @@ public void testRestoreIndexWithMissingShards() throws Exception {
 
     public void testRestoreIndexWithShardsMissingInLocalGateway() throws Exception {
         logger.info("--> start 2 nodes");
-        Settings nodeSettings = Settings.builder()
-                .put(EnableAllocationDecider.CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING.getKey(), EnableAllocationDecider.Rebalance.NONE)
-                .build();
 
-        internalCluster().startNodes(2, nodeSettings);
+        internalCluster().startNodes(2);
         cluster().wipeIndices("_all");
 
         createRepository("test-repo", "fs");
@@ -576,6 +572,10 @@ public void testRestoreIndexWithShardsMissingInLocalGateway() throws Exception {
 
         indexRandomDocs("test-idx", 100);
 
+        logger.info("--> force merging down to a single segment to get a deterministic set of files");
+        assertEquals(client().admin().indices().prepareForceMerge("test-idx").setMaxNumSegments(1).setFlush(true).get().getFailedShards(),
+            0);
+
         createSnapshot("test-repo", "test-snap-1", Collections.singletonList("test-idx"));
 
         logger.info("--> close the index");