Update functional_testing.rst

Make it clear that the tests.py file in the functional_testing directory is the one to edit. Also, warn about a potential "gotcha": if the tests.py file is executable, it will be silently ignored. (I ran into this problem while going through the tutorial. This can happen if the file is being edited on a network file share from a different OS, for example.).
Update functional_testing.rst

Wrap long line.
Merge pull request #1423 from deisner/patch-1

Update functional_testing.rst