Remove image backends config from runtests.py

It is not needed since Pillow is the default and get_backend works with
full package name.
Refactor backend names to pillow and wand

This uses absolute_import and refactors the backend names to pillow &
wand. It changes tests to use these names and also modified the test
runner to not pass any parameters concerning the image backends since the
defaults are enough for tests. It also fixes a minor bug that was occured
when the image file had already been read (multiple renditions in a single
page)