Add test for [text](<url\>)

[andersk]

From commonmark/commonmark.js#158: there’s a discrepancy between cmark and commonmark.js on [text](<url\>). cmark gives

<p>[text](&lt;url&gt;)</p>

while commonmark.js previously gave

<p><a href="url%5C">text</a></p>

and was just fixed in Git to give

<p><a href="%3Curl%3E">text</a></p>

which seems most consistent with the spec.

[jgm]

Brought cmark into conformance.