The sytax tree output files now use .rast extension
(rust-analyzer syntax tree or rust abstract syntax tree
(whatever)).
This format has a editors/code/ra_syntax_tree.tmGrammar.json declaration
that supplies nice syntax highlighting for .rast files.
As of rust-lang/rust#61319 the correct order for functions that are both
unsafe and async is: `async unsafe fn` and not `unsafe async fn`.
This commit updates the parser tests to reflect this, and corrects
parsing behavior to accept the correct ordering.
Fixes#3025
