Tests: Fix SearchFieldsIT.testDocValueFields (elastic#31995)

This test produced different implementations of joda time classes,
depending on if the data was serialized or not (DateTime vs
MutableDateTime). This now uses a common base class to extract the
milliseconds from the data.

Closes elastic#31992

server/src/test/java/org/elasticsearch/search/fields/SearchFieldsIT.java

@@ -46,8 +46,7 @@
 import org.elasticsearch.search.sort.SortOrder;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.test.InternalSettingsPlugin;
-import org.joda.time.DateTime;
-import org.joda.time.DateTimeZone;
+import org.joda.time.base.BaseDateTime;
 
 import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
@@ -702,7 +701,6 @@ public void testSingleValueFieldDatatField() throws ExecutionException, Interrup
         assertThat(fields.get("test_field").getValue(), equalTo("foobar"));
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/31992")
     public void testDocValueFields() throws Exception {
         createIndex("test");
 
@@ -804,8 +802,8 @@ public void testDocValueFields() throws Exception {
         assertThat(searchResponse.getHits().getAt(0).getFields().get("long_field").getValue(), equalTo((Object) 4L));
         assertThat(searchResponse.getHits().getAt(0).getFields().get("float_field").getValue(), equalTo((Object) 5.0));
         assertThat(searchResponse.getHits().getAt(0).getFields().get("double_field").getValue(), equalTo((Object) 6.0d));
-        assertThat(searchResponse.getHits().getAt(0).getFields().get("date_field").getValue(),
-            equalTo(new DateTime(date.toInstant().toEpochMilli(), DateTimeZone.UTC)));
+        BaseDateTime dateField = searchResponse.getHits().getAt(0).getFields().get("date_field").getValue();
+        assertThat(dateField.getMillis(), equalTo(date.toInstant().toEpochMilli()));
         assertThat(searchResponse.getHits().getAt(0).getFields().get("boolean_field").getValue(), equalTo((Object) true));
         assertThat(searchResponse.getHits().getAt(0).getFields().get("text_field").getValue(), equalTo("foo"));
         assertThat(searchResponse.getHits().getAt(0).getFields().get("keyword_field").getValue(), equalTo("foo"));