Test pt-upgrade host-to-host. Implement diff_query_times(). Export $test_diff from PerconaTest instead of doing diag() in no_diff().

t/pt-upgrade/samples/003/host2.sql

@@ -0,0 +1,5 @@
+USE test;
+
+-- Increase column size from 8 to 16 so insert.log causes
+-- a warning diff (value is truncated on host1).
+ALTER TABLE t CHANGE COLUMN username username varchar(16) default NULL;