Adds 2 tests for Flask.run method

I saw `Flask.run` method was untested and I think a couple of tests wouldn't hurt (and increases code coverage). I would love to hear some feedback about it even if it is not merged at all. I feel pretty enthusiastic about contributing to Flask since I'm a heavy user of the framework.
Please do that, and don't forget to use ``try...finally`` for this. Meanwhile i see if i can port the testsuite...