Flask: Adding missing Flask-Testing dependency

Following the guidelines in [README.rst](flask/README.rst), there seems to be a missing dependency in the requirements.txt file, which causes the tests to fail:

```python
$ python tests.py 
Traceback (most recent call last):
  File "tests.py", line 9, in <module>
    from flask.ext.testing import TestCase
  File "/home/benno/.virtualenvs/notejam/local/lib/python2.7/site-packages/flask/exthook.py", line 86, in load_module
    raise ImportError('No module named %s' % fullname)
ImportError: No module named flask.ext.testing
```

Adding `Flask-Testing` as a dependency solved this.