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.
The problem with this is that you now have this method patched in some other test, and that test might rely on the actual behavior of run_simple.

The testsuite should get ported to py.test anyways, which should provide tools for monkeypatching.