Fix ui test.

This test checks if the compiler complains about accesing a private
field before complaining (or crashing) about the private function on it
not marked as stable/unstable.

The interface of the internal type (sys_common's Mutex) used for this
was changed. With this change, it uses another function to test for the
same issue.

Author: m-ou-se

src/test/ui/issues/issue-54062.rs

@@ -7,7 +7,6 @@ struct Test {
 fn main() {}
 
 fn testing(test: Test) {
-    let _ = test.comps.inner.lock().unwrap();
+    let _ = test.comps.inner.try_lock();
     //~^ ERROR: field `inner` of struct `Mutex` is private
-    //~| ERROR: no method named `unwrap` found
 }

src/test/ui/issues/issue-54062.stderr

@@ -1,16 +1,9 @@
 error[E0616]: field `inner` of struct `Mutex` is private
   --> $DIR/issue-54062.rs:10:24
    |
-LL |     let _ = test.comps.inner.lock().unwrap();
+LL |     let _ = test.comps.inner.try_lock();
    |                        ^^^^^ private field
 
-error[E0599]: no method named `unwrap` found for struct `std::sys_common::mutex::MutexGuard<'_>` in the current scope
-  --> $DIR/issue-54062.rs:10:37
-   |
-LL |     let _ = test.comps.inner.lock().unwrap();
-   |                                     ^^^^^^ method not found in `std::sys_common::mutex::MutexGuard<'_>`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0599, E0616.
-For more information about an error, try `rustc --explain E0599`.
+For more information about this error, try `rustc --explain E0616`.