As setUp is run for each test, the previous implementation was running pelican
twice (once for relative urls and once for absolute) for each test, which was not
really optimal :-).
Solution: make a base class which is inherated by a class for relative urls and
another for absolute urls.
Set the ASSET_URL to be relative to the 'theme/' url, which requires to use
{{ SITEURL }}/{{ ASSET_URL }} in the template and make it works with both
relative and absolute urls.
