[TEST] reduce the number of snapshotted shards to 1 in testSnapshotSucceedsAfterSnapshotFailure()

so that we are more likely to trigger I/O exceptions on writing the control files during the
finalize phase of snapshotting (with the aim of triggering an I/O failure when writing pending-index-*).

core/src/test/java/org/elasticsearch/snapshots/SharedClusterSnapshotRestoreIT.java

@@ -2683,7 +2683,11 @@ public class SharedClusterSnapshotRestoreIT extends AbstractSnapshotIntegTestCas
                     .put("random", randomAsciiOfLength(10))));
 
         logger.info("--> indexing some data");
-        createIndex("test-idx");
+        assertAcked(prepareCreate("test-idx").setSettings(
+            // the less the number of shards, the less control files we have, so we are giving a higher probability of
+            // triggering an IOException toward the end when writing the pending-index-* files, which are the files
+            // that caused problems with writing subsequent snapshots if they happened to be lingering in the repository
+            Settings.builder().put(SETTING_NUMBER_OF_SHARDS, 1).put(SETTING_NUMBER_OF_REPLICAS, 0)));
         ensureGreen();
         final int numDocs = randomIntBetween(1, 5);
         for (int i = 0; i < numDocs; i++) {