Fix SimpleValidateQueryIT#testExplainValidateQueryTwoNodes (#84296) (#84312)

Under rare circumstances the 10 documents the test indexes might land on only
one shard, resulting in a different output for the validation query than
expected. By adding fixed doc ids we can avoid this and also make the test more
reproducable.

Closes #84218

Author: Christoph Büscher

server/src/internalClusterTest/java/org/elasticsearch/validate/SimpleValidateQueryIT.java

@@ -183,7 +183,11 @@ public void testExplainValidateQueryTwoNodes() throws IOException {
             .actionGet();
 
         for (int i = 0; i < 10; i++) {
-            client().prepareIndex("test").setSource("foo", "text", "bar", i, "baz", "blort").execute().actionGet();
+            client().prepareIndex("test")
+                .setSource("foo", "text", "bar", i, "baz", "blort")
+                .setId(Integer.toString(i))
+                .execute()
+                .actionGet();
         }
         refresh();