Add failing test where attribute closes the quotations prematurely

[kamal]

When an attribute contains newlines, a mutation to a MustacheStatement inside of it will cause printing the AST to prematurely close the quotations, causing invalid markup.

Example,

<div
  class="
    block
    {{if this.foo "bar"}}
  "
>
  hello
</div>

when parsed, mutated and printed will produce

<div
  class=""
    block
    {{if this.foo "bar"}}
  "
>
  hello
</div>

[dcyriller]

I am closing this PR as the failing test is fixed in #665