If a & simple selector is on a style rule with no parent style rule,
then it behaves like :scope - but notably, :scope provides 1
specificity in the class category, but & is supposed to provide 0.
To solve this, we stop replacing it directly, and just handle the & like
any other simple selector. We know that if the selector engine ever
sees one, it's equivalent to :scope, because the nested ones will have
been replaced with :is() before that point.
This gets us one more subtest pass. :^)
A few are skipped for now:
- A few ref tests fail
- Crash tests are not supported by our runner and time out
- top-level-is-scope.html crashes and needs further investigation
