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.).
@deisner Please wrap this line to 79 characters, and I will merge it. Thank you!