[Tests] Fix ReopenWhileClosingIT with correct min num shards

The test intercepts TransportVerifyShardBeforeCloseAction shard
requests, so it needs a minimum of 2 primary shards on 2 different
 nodes to correctly intercepts requests.

server/src/test/java/org/elasticsearch/indices/state/ReopenWhileClosingIT.java

@@ -58,6 +58,11 @@ protected Collection<Class<? extends Plugin>> nodePlugins() {
         return singletonList(MockTransportService.TestPlugin.class);
     }
 
+    @Override
+    protected int minimumNumberOfShards() {
+        return 2;
+    }
+
     public void testReopenDuringClose() throws Exception {
         final String indexName = "test";
         createIndexWithDocs(indexName);