[Tests] Fix slowness of AutoDateHistogramAggregatorTests (#35072)

Randomize test assertion and test set size instead of asserting on an
exhaustive list of dates with fixed test set size. Also refactor common 
objects used to avoid recreating them, avoid date to string conversion
and reduce duplicate test code

Closes #33181

server/src/main/java/org/elasticsearch/search/aggregations/bucket/histogram/package-info.java

@@ -0,0 +1,23 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/**
+ * Histogram module for different bucket specifications used in aggregation.
+ */
+package org.elasticsearch.search.aggregations.bucket.histogram;