Escape angle brackets in serializer attribute tests

[EmilStenstrom]

Summary

• escape < and > in serializer attribute-value expectations
• align serializer tests with the HTML serialization change from Escape "<" and ">" in attributes when serializing HTML whatwg/html#6235

Fixes #191.

[EmilStenstrom]

Note:

• The nokogiri and parse5 failures below are related to the selectedcontent changes, I see no failures connected to my change above, which points to the fact that serializers tests are not run for those parsers.
• No failures for html5gum and skeleton points to no serializer tests being run on those platforms either.

[EmilStenstrom]

@annevk Mind reviewing this as well? :)

[annevk]

Are these tests not run in browsers? I didn't notice these when fixing the behavior in WebKit. @zcorpan do you know?

[EmilStenstrom]

@annevk What's the right process to get a merge here?