make #79 pull request more robust

[mcepl]

@jorgenschaefer I have probably screwed up something, but your tests from #79 seem to be a little bit too fragile:

======================================================================
ERROR: test_does_not_fail_for_permission_denied (ropetest.projecttest.ProjectTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/matej/archiv/knihovna/repos/rope/ropetest/projecttest.py", line 228, in test_does_not_fail_for_permission_denied
    os.makedirs(bad_dir)
  File "/usr/lib64/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 17] File exists: 'sample_folder/bad_dir'

----------------------------------------------------------------------

Could you make your tests a bit more robust, please, and survive some files existing?

[mcepl]

Actually, it is even worse ... your tests are leaving sample_folder/bad_dir/ behind. Please, make the test to clean up after itself.

[jorgenschaefer]

See #85.

Is there a particular reason why ProjectTest leaves sample_folder lying around after the test run? It could simple rmtree that folder and thus clean up for all tests in one place.

[mcepl]

Fixed in #85

[mcepl]

Actually, making cleanup more automagical is not a bad idea. @aligrudi any ideas, where to put it?