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.
Your IDE has added quite a lot of stuff to the project. You can edit ``.git/info/exclude`` to automatically ignore these files.