Use DROP USER in test

DROP USER existed in 5.6
http://www.asktheway.org/official-documents/mysql/refman-5.6-en.html-chapter/sql-statements.html#drop-user

5.7 and MariaDB versions support an IF EXISTS option.
https://jira.mariadb.org/browse/MDEV-7288
Using executable comment syntax we can make it compatible.

When using DROP USER, like all user modifications, FLUSH PRIVILEGES
isn't required.

Author: grooverdan

src/conn/mod.rs

@@ -1552,10 +1552,9 @@ mod test {
                 .map(|x| ((x % (123 - 97)) + 97) as char)
                 .collect();
 
-            conn.query_drop("DELETE FROM mysql.user WHERE user = '__mats'")
+            conn.query_drop("DROP USER /*!50700 IF EXISTS */ /*M!100103 IF EXISTS */ __mats")
                 .await
                 .unwrap();
-            conn.query_drop("FLUSH PRIVILEGES").await.unwrap();
 
             if conn.inner.is_mariadb || conn.server_version() < (5, 7, 0) {
                 if matches!(conn.server_version(), (5, 6, _)) {
@@ -1587,8 +1586,6 @@ mod test {
                 .unwrap();
             };
 
-            conn.query_drop("FLUSH PRIVILEGES").await.unwrap();
-
             let mut conn2 = Conn::new(get_opts().secure_auth(false)).await.unwrap();
             conn2
                 .change_user(