add an unfixable case

as recommended in
https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/problem.20when.20suggestion.20only.20removes.20one.20.22level.22.20of.20a.20lint/near/532894687

Author: ada4a

tests/ui/mut_mut_unfixable.rs

@@ -31,4 +31,12 @@ fn main() {
         //~^ mut_mut
         ***y + **x;
     }
+
+    if fun(x) {
+        // The lint will remove the extra `&mut`, but the result will still be a `&mut` of an expr
+        // of type `&mut _` (x), so the lint will fire again. That's because we've decided that
+        // doing both fixes in one run is not worth it, given how improbable code like this is.
+        let y = &mut &mut x;
+        //~^ mut_mut
+    }
 }

tests/ui/mut_mut_unfixable.stderr

@@ -31,5 +31,11 @@ error: an expression of form `&mut &mut _`
 LL |         let y = &mut &mut &mut 2;
    |                 ^^^^^^^^^^^^^^^^ help: remove the extra `&mut`s: `&mut 2`
 
-error: aborting due to 5 previous errors
+error: an expression of form `&mut &mut _`
+  --> tests/ui/mut_mut_unfixable.rs:39:17
+   |
+LL |         let y = &mut &mut x;
+   |                 ^^^^^^^^^^^ help: remove the extra `&mut`: `&mut x`
+
+error: aborting due to 6 previous errors