Fix bug in image_test

The final assert creates a list of booleans. They intended to
test whether a file had been uploaded, but didn't since.
assert [False, False, False] doesn't raise an error.
Pylint fixes
Allow test functions to have longer names.
Make tests skippable where appropriate

This was impossible before because skips were first
introduced to unittest in python version 2.7 and
PRAW is designed to work with python 2.6+.