want a way to make a TestBrowser logged in as a particular user

I'd like to use a TestBrowser to examine what happens when I'm logged in as a member of ~admins. According to lifeless, there is no builtin way to do this: you would think you could call BrowserTestCase.getUserBrowser(url, user=user), but that seems not to work because it defaults to anonymous if no password is provided.

At the moment in lp:~mbp/launchpad/flags-gui I have this:

    def getFeaturePageBrowserAsAdmin(self):
        root = getUtility(ILaunchpadRoot)
        url = canonical_url(root, view_name='+feature-rules')
        # XXX: To make a UserBrowser, you must know the password. This should
        # be separated out into test infrastructure -- mbp 20100923.
        user = self.factory.makePerson(password='test')
        admin_team = getUtility(ILaunchpadCelebrities).admin
        with person_logged_in(admin_team.teamowner):
            admin_team.addMember(user, reviewer=admin_team.teamowner)
        return self.getUserBrowser(url, user=user, password='test')

Perhaps there's a builtin way to do it or perhaps this can be lifted into somewhere else.