tests: don't use jinja2.ext.with_ in tests

This extension is now builtin and no longer exist under the old name.
Since the test was just verifying that we can load extensions and uses
other extensions, we can just remove it from the test.

slapos/recipe/template/README.jinja2.txt

@@ -319,7 +319,6 @@ whitespace-separated. By default, none is loaded.
     ... context = key bar buildout:parts
     ... # We don't actually use all those extensions in this minimal example.
     ... extensions = jinja2.ext.do jinja2.ext.loopcontrols
-    ...   jinja2.ext.with_
     ... ''')
     >>> run_buildout()
     Develop: '/sample-buildout/.'