Nested attribute selector without & is parsed as raw

[bartveneman]

When parsing this (relaxed nesting) CSS we end up with a Raw node inside the first Rule, instead of another Rule:

a {
  [test] {}
}

• Link to reproduction: https://stackblitz.com/edit/csstree-nested-attr-selector-repro?file=index.js
• CodePen showing that it actually works in a browser https://codepen.io/bartveneman/pen/XWOQLRz

When adding a & in front the parsing seems to work:

a {
  & [test] {}
}

Maybe there's more relaxed nesting rules that don't work, but this is the only I've spotted so far.