Fix test failure (timeout) in --valgrind tests in Buildbot.

The main.ps_ddl test does SELECT * FROM mysql.general_log; that can be really
expensive with --valgrind if previous test cases put lots of data in the
general log since last server restart. Fix by truncating the log at test start.

mysql-test/r/ps_ddl.result

@@ -4,6 +4,7 @@ drop procedure if exists p_verify_reprepare_count;
 drop procedure if exists p1;
 drop function if exists f1;
 drop view if exists v1, v2;
+TRUNCATE TABLE mysql.general_log;
 create procedure p_verify_reprepare_count(expected int)
 begin
 declare old_reprepare_count int default @reprepare_count;

mysql-test/t/ps_ddl.test

@@ -58,6 +58,10 @@ drop function if exists f1;
 drop view if exists v1, v2;
 --enable_warnings
 
+# Avoid selecting from a huge table possibly left over from previous tests,
+# as this really hurts --valgrind testing.
+TRUNCATE TABLE mysql.general_log;
+
 delimiter |;
 create procedure p_verify_reprepare_count(expected int)
 begin