Serializer tests do not quote `<` and `>` in attributes

[EmilStenstrom]

The change in whatwg/html#6235 meant that browsers are now quoting < and > characters in attributes.

But serializer tests do not quote attributes:

{"description": "proper attribute value non-quoting (with <)",
 "input": [["StartTag", "http://www.w3.org/1999/xhtml", "span", [{"namespace": null, "name": "title", "value": "foo<bar"}]]],
 "expected": ["<span title=foo<bar>"],
 "xhtml":    ["<span title=\"foo&lt;bar\">"]
},

and here...

{"description": "proper attribute value quoting (with >)",
 "input": [["StartTag", "http://www.w3.org/1999/xhtml", "span", [{"namespace": null, "name": "title", "value": "foo>bar"}]]],
 "expected": ["<span title=\"foo>bar\">"]
},

Would you be open to a PR changing these tests?