Hi Erlis,
tSQLt doesn't currently have this functionality. We've been waiting for someone to ask for it ;-)
Last last year, I had done a proof of concept to see how hard it would be. I've attached the script that implements that proof of concept here. However, I never fully tested it, and the code is a little old - therefore, I can't confidently say it works in the current release of tSQLt. If you want, you can try it out.
What this script does is add a Start and End time column to the TestResult table and then value those two columns in when tests are executed. This code only modifies the default result formatter - in other words, it would only work when you execute tSQLt.RunAll in Management Studio. It will not display the results in the XML output or in SQL Test.
Hopefully this helps you troubleshoot you're problem!