fix tests, now they all run, pep8-ed test_ldiftree

I did a bit of refactoring of the ldaptor/test/test_ldiftree.py so now that those 3 failing tests now pass. This opened up 50 additional tests that also pass.

It looks like the code was written around python 2.4 so I cleaned it up a bit, removed deprecated 'sets' and pep8-ed as much as I could.

I recommend that after this commit, we branch with 0.54.0 and tag for release.

Update:  less pep8-ed then before to keep with twisted standards.