Nikola not failing TestCheckFailure

test_integration.py

class TestCheckFailure(DemoBuildTest):
    """The demo build should pass 'nikola check'"""

    def test_check_links_fail(self):
        with cd(self.target_dir):
            os.unlink(os.path.join("output", "archive.html"))

            __main__.main(['check', '-l'])

    def test_check_files_fail(self):
        with cd(self.target_dir):
            with io.open(os.path.join("output", "foobar"), "w+", encoding="utf8") as outf:
                outf.write("foo")

            __main__.main(['check', '-f'])