[testlib] consider rset for template / view selection (closes #2447183)
The original code did something like:
kwargs['rset'] = rset
viewfunc = lambda **k: viewsreg.main_template(req, template, **kwargs)
kwargs.pop('rset')
return self._test_view(viewfunc, view, template, kwargs)
meaning that by the time ``viewfunc`` (i.e. the lambda function) is called,
``rset`` was popped from ``kwargs`` and had no chance to be passed to ``main_template``.
