Add fixture for attributes

Author: wooorm

test/fixtures/attributes/index.html

@@ -0,0 +1,9 @@
+<p id="foo" class="bar baz" data-qux="quux"></p>
+
+<p data-123="456"></p>
+
+<img alt hidden>
+
+<a download></a>
+
+<a download=example.mp3></a>