Testlib: Add test duration to xml output

[ChangeLog][QtTest] Added test duration to xml output.
When running tests with xml output a new tag of the form
<duration msecs="123"/> is added to each test function and the test as a
whole.

Change-Id: Ibc4db066b6acf5fac6c578f5e5ca5ce4b5d8ea8e
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Jason McDonald <macadder1@gmail.com>