Slight refactoring of the testcase for Tester.run()

This is a slight refactoring of the test case for the utils.Tester.run()
method which removes code duplicate and adds one extra assertion (the method
is a really central one for Rally and should be carefully tested).

Blueprint test-engine

Change-Id: Ibcef9637b17d181dd4cff5656d510de24a600a1c