Fix new unused_mut detected by nightly-2023-04-30

    warning: variable does not need to be mutable
      --> src/expand.rs:51:17
       |
    51 |             let mut prev = &mut self.vec[i];
       |                 ----^^^^
       |                 |
       |                 help: remove this `mut`
       |
       = note: `#[warn(unused_mut)]` on by default

    warning: variable does not need to be mutable
       --> src/run.rs:649:21
        |
    649 |                 let mut entry = map
        |                     ----^^^^^
        |                     |
        |                     help: remove this `mut`

src/expand.rs

@@ -48,7 +48,7 @@ impl ExpandedTestSet {
 
     fn insert(&mut self, test: Test, error: Option<Error>, is_from_glob: bool) {
         if let Some(&i) = self.path_to_index.get(&test.path) {
-            let mut prev = &mut self.vec[i];
+            let prev = &mut self.vec[i];
             if prev.is_from_glob {
                 prev.test.expected = test.expected;
                 return;

src/run.rs

@@ -646,7 +646,7 @@ fn parse_cargo_json(
                     Some(test) => test,
                     None => continue,
                 };
-                let mut entry = map
+                let entry = map
                     .entry(de.target.src_path)
                     .or_insert_with(Stderr::default);
                 if de.message.level == "error" {