TokenStream holds a `tt::Subtree` but assumes its `delimiter` is always
`None`. In particular, the iterator implementation iterates over the
inner `token_trees` and ignores the `delimiter`.
However, `TokenStream::from_str` violated this assumption when the input
consists of a single Group by producing a Subtree with an outer
delimiter, which was ignored as seen by a procedural macro.
In this case, wrap an extra level of Subtree around it.
Fixes#7810Fixes#7875
