Missing coverage for `<button><p></button>x`

[zcorpan]

button is special in relation to p (in that </button> generates implied end tags),
...
I don't see a test for <button><p></button>x, so that is missing coverage.

Originally posted by @zcorpan in #142 (comment)

An end tag whose tag name is one of: "address", "article", "aside", "blockquote", "button", "center", "details", "dialog", "dir", "div", "dl", "fieldset", "figcaption", "figure", "footer", "header", "hgroup", "listing", "main", "menu", "nav", "ol", "pre", "section", "summary", "ul"

https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inbody