test: link and title with newline

[ChristianMurphy]

[](a (a 
))

throws

thread panicked at 'expected link', src/subtokenize.rs:148:57

markdown-rs/src/subtokenize.rs

Line 148 in 56cd834

let link_curr = enter.link.as_ref().expect("expected link");

when in CommonMark it is valid https://spec.commonmark.org/dingus/?text=%5B%5D(a%20(a%20%0A))
and should render:

<p><a href="a" title="a 
"></a></p>