Disable these EXPLAIN tests if using mysql2 and prepared statements are enabled

This is because these tests will never pass, due to the EXPLAIN output
of MySQL always showing the final result of the query without the binds.

```
Failure:
ExplainTest#test_relation_explain_with_last [test/cases/explain_test.rb:108]:
Expected /SELECT\ `cars`\.\*\ FROM\ `cars`\ ORDER\ BY\ `cars`\.`id`\ DESC\ LIMIT\ \?/ to match EXPLAIN SELECT `cars`.* FROM `cars` ORDER BY `cars`.`id` DESC LIMIT 1
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+---------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra               |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+---------------------+
|  1 | SIMPLE      | cars  | NULL       | index | NULL          | PRIMARY | 8       | NULL |    1 |    100.0 | Backward index scan |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+---------------------+
1 row in set (0.00 sec)
.

bin/test test/cases/explain_test.rb:102

F

Failure:
ExplainTest#test_relation_explain_with_first [test/cases/explain_test.rb:99]:
Expected /SELECT\ `cars`\.\*\ FROM\ `cars`\ ORDER\ BY\ `cars`\.`id`\ ASC\ LIMIT\ \?/ to match EXPLAIN SELECT `cars`.* FROM `cars` ORDER BY `cars`.`id` ASC LIMIT 1
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------+
|  1 | SIMPLE      | cars  | NULL       | index | NULL          | PRIMARY | 8       | NULL |    1 |    100.0 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------+
1 row in set (0.00 sec)
.

bin/test test/cases/explain_test.rb:93
```

Author: zzak

activerecord/test/cases/explain_test.rb

@@ -89,22 +89,26 @@ def test_relation_explain_with_sum
       assert_match(expected_query, message)
     end
 
-    def test_relation_explain_with_first
-      expected_query = capture_sql {
-        Car.all.first
-      }.first
-      message = Car.all.explain.first
-      assert_match(/^EXPLAIN/, message)
-      assert_match(expected_query, message)
-    end
+    unless current_adapter?(:Mysql2Adapter) && ActiveRecord::Base.lease_connection.prepared_statements
+      def test_relation_explain_with_first
+        expected_query = capture_sql {
+          Car.all.first
+        }.first
+        message = Car.all.explain.first
+        assert_match(/^EXPLAIN/, message)
+        puts expected_query
+        puts message
+        assert_match(expected_query, message)
+      end
 
-    def test_relation_explain_with_last
-      expected_query = capture_sql {
-        Car.all.last
-      }.first
-      message = Car.all.explain.last
-      assert_match(/^EXPLAIN/, message)
-      assert_match(expected_query, message)
+      def test_relation_explain_with_last
+        expected_query = capture_sql {
+          Car.all.last
+        }.first
+        message = Car.all.explain.last
+        assert_match(/^EXPLAIN/, message)
+        assert_match(expected_query, message)
+      end
     end
 
     def test_relation_explain_with_pluck