Bug#27934 test client_xml misssing initialization

Problem: missing initialization, if the previous test
fails leaving table t1, client_xml fails as well.
Fix: adding initialization.

mysql-test/r/client_xml.result

+drop table if exists t1;
 create table t1 (
 `a&b` int,
 `a<b` int,

mysql-test/t/client_xml.test

 # Can't run with embedded server
 -- source include/not_embedded.inc
 
+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+
 # Test of the xml output of the 'mysql' and 'mysqldump' clients -- makes
 # sure that basic encoding issues are handled properly
 create table t1 (