Disable warnings when dropping databases

Dropping a database with DROP DATABASE ... IF NOT EXISTS will cause
warnings to be logged if it doesn't exist. The masking_user test was
missing the part that disables these warnings.

maxscale-system-test/masking/masking_user/t/masking_user.test

@@ -2,8 +2,9 @@
 # Masking User
 #
 # See ../masking_rules.json
-
+--disable_warnings
 drop database if exists maskingdb;
+--enable_warnings
 
 create database maskingdb;
 use maskingdb;