Fix IndexShardIT.testIndexCanChangeCustomDataPath() (#43978)

The test IndexShardIT.testIndexCanChangeCustomDataPath() fails
 on 7.x and 7.3 because the translog cannot be recovered.

While I can't reproduce the issue, I think it has been introduced in #43752 
which changed ReadOnlyEngine so that it opens the translog in its 
constructor in order to load the translog stats. This opening writes a 
new checkpoint file, but because 7.x/7.3 does not wait for shards to be 
started after being closed, the test immediately starts to copy shard files
 to a new directory and possibly does not copy all the required translog files.

By waiting for the shards to be started after being closed, we ensure 
that the shards (and engines) have been correctly initialized and that
 the translog checkpoint file is not currently being written.

closes #43964

server/src/test/java/org/elasticsearch/index/shard/IndexShardIT.java

@@ -30,6 +30,7 @@
 import org.elasticsearch.action.index.IndexResponse;
 import org.elasticsearch.action.search.SearchRequest;
 import org.elasticsearch.action.search.SearchResponse;
+import org.elasticsearch.action.support.ActiveShardCount;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.cluster.ClusterInfoService;
 import org.elasticsearch.cluster.ClusterState;
@@ -296,7 +297,7 @@ public void testIndexCanChangeCustomDataPath() throws Exception {
         assertHitCount(client().prepareSearch(index).setSize(0).get(), 1L);
 
         logger.info("--> closing the index [{}]", index);
-        assertAcked(client().admin().indices().prepareClose(index));
+        assertAcked(client().admin().indices().prepareClose(index).setWaitForActiveShards(ActiveShardCount.DEFAULT));
         logger.info("--> index closed, re-opening...");
         assertAcked(client().admin().indices().prepareOpen(index));
         logger.info("--> index re-opened");
@@ -306,7 +307,7 @@ public void testIndexCanChangeCustomDataPath() throws Exception {
 
         // Now, try closing and changing the settings
         logger.info("--> closing the index [{}] before updating data_path", index);
-        assertAcked(client().admin().indices().prepareClose(index));
+        assertAcked(client().admin().indices().prepareClose(index).setWaitForActiveShards(ActiveShardCount.DEFAULT));
 
         final Path newIndexDataPath = sharedDataPath.resolve("end-" + randomAlphaOfLength(10));
         IOUtils.rm(newIndexDataPath);