add timestamp attribute to test suite

it supports timestamp of start time in test suite out of Apache ant task [XMLJunitResultFormatter](http://svn.apache.org/viewvc/ant/core/trunk/src/main/org/apache/tools/ant/taskdefs/optional/junit/XMLJUnitResultFormatter.java?view=markup). see line 136 ~ 139.

This attribute is very helpful for tracing test performance.