Explicitly set mappings in SearchAfterIT (#54262)

We have some very occasional failures in SearchAfterIT, where a search throws
an exception because a shard does not have the mapping for the requested sort
field. The field should have been added in a dynamic mapping update after an
index event, but it seems that there can sometimes be a small delay in propagating
this update to the shards.

This commit changes the test to explicitly define the relevant field at index creation
time.

Fixes #51900

server/src/test/java/org/elasticsearch/search/searchafter/SearchAfterIT.java

@@ -51,7 +51,7 @@ public class SearchAfterIT extends ESIntegTestCase {
 
     public void testsShouldFail() throws Exception {
         assertAcked(client().admin().indices().prepareCreate("test")
-            .addMapping("type1", "field2", "type=keyword")
+            .addMapping("type1", "field1", "type=long", "field2", "type=keyword")
             .get()
         );
         ensureGreen();