Make report in form of two notebooks.

        Use nbdime diff-web to present the difference between reference
        cells and test cells.

def make_report(self, outcome):
        """Make report in form of two notebooks.

        Use nbdime diff-web to present the difference between reference
        cells and test cells.
        """
        failures = self.getreports('failed')
        if not failures:
            return
        for rep in failures:
            # Check if this is a notebook node
            msg = self._getfailureheadline(rep)
            lines = rep.longrepr.splitlines()
            if len(lines) > 1:
                self.section(msg, lines[1])
            self._outrep_summary(rep)
        tmpdir = tempfile.mkdtemp()
        try:
            ref_file = os.path.join(tmpdir, 'reference.ipynb')
            test_file = os.path.join(tmpdir, 'test_result.ipynb')
            with io.open(ref_file, "w", encoding="utf8") as f:
                nbformat.write(self.nb_ref, f)
            with io.open(test_file, "w", encoding="utf8") as f:
                nbformat.write(self.nb_test, f)
            run_server(
                port=0,     # Run on random port
                cwd=tmpdir,
                closable=True,
                on_port=lambda port: browse(
                    port, ref_file, test_file, None))
        finally:
            shutil.rmtree(tmpdir)