Remove duplicate tests in test_contents
Remove duplicate tests in test_contents

TestArticle was a subclass of TestPage and therefore included all
the tests TestPage already had. Effectively tests from TestPage
were run twice. This splits the common parts of TestPage to a
base class called TestBase and TestPage/TestArticle will both
inherit from it.

avaris/pelican,ingwinlu/pelican,ingwinlu/pelican,getpelican/pelican,farseerfc/pelican,getpelican/pelican,farseerfc/pelican,avaris/pelican