Add a method to modify the global xwiki configuration in selenium tests

from irc:
<vmassol_> I think we haven't done a lot of tests on the different configuration combination but we need to do that
<vmassol_> I think that could be acceptable if we create a new API in AbstractXWikiTestCase for this
<vmassol_> like setXWikiConfigurationProperty()
<vmassol_> we would execute the code in preview mode only
<vmassol_> so that no doc is created
<vmassol_> you could use a special page like
<vmassol_> Test.XWikiConfigurationPageForTest