test: add test for pages

[curbengh]

in preparation of #59.

[curbengh]

I will split

    const result = generator(locals);

    result.path.should.eql('sitemap.xml');
    result.data.should.eql(sitemapTmpl.render({
      config: hexo.config,
      posts: posts.toArray().concat(pages.toArray())
    }));

    const $ = cheerio.load(result.data);
    const allPosts = Object.assign({}, posts.data.concat(pages.data));

    $('url').each((index, element) => {
      $(element).children('loc').text().should.eql(allPosts[index].permalink);
      $(element).children('lastmod').text().should.eql(allPosts[index].updated.toISOString());
    });

into two tests in a separate PR.

[coveralls]

Coverage remained the same at 100.0% when pulling 816c406 on curbengh:page into c5f7c55 on hexojs:master.

[curbengh]

rebased