Update test suite to nightly-2024-01-09

    warning: variable does not need to be mutable
      --> tests/common/parse.rs:28:17
       |
    28 |             Err(mut diagnostic) => {
       |                 ----^^^^^^^^^^
       |                 |
       |                 help: remove this `mut`
       |
       = note: `#[warn(unused_mut)]` on by default

    warning: variable does not need to be mutable
       --> tests/test_round_trip.rs:109:21
        |
    109 |                 Err(mut diagnostic) => {
        |                     ----^^^^^^^^^^
        |                     |
        |                     help: remove this `mut`

tests/common/parse.rs

@@ -25,7 +25,7 @@ pub fn librustc_expr(input: &str) -> Option<P<ast::Expr>> {
         .parse_expr();
         match e {
             Ok(expr) => Some(expr),
-            Err(mut diagnostic) => {
+            Err(diagnostic) => {
                 diagnostic.emit();
                 None
             }

tests/test_round_trip.rs

@@ -106,7 +106,7 @@ fn test(path: &Path, failed: &AtomicUsize, abort_after: usize) {
             };
             let after = match librustc_parse(back, &sess) {
                 Ok(after) => after,
-                Err(mut diagnostic) => {
+                Err(diagnostic) => {
                     errorf!("=== {}: librustc failed to parse", path.display());
                     diagnostic.emit();
                     return Err(false);