Fix clippy test output

Author: estebank

src/tools/clippy/tests/ui/const_is_empty.stderr

@@ -157,11 +157,17 @@ error: this expression always evaluates to true
 LL |     let _ = val.is_empty();
    |             ^^^^^^^^^^^^^^
 
+error: this expression always evaluates to true
+  --> tests/ui/const_is_empty.rs:181:17
+   |
+LL |         assert!(EMPTY_STR.is_empty());
+   |                 ^^^^^^^^^^^^^^^^^^^^
+
 error: this expression always evaluates to true
   --> tests/ui/const_is_empty.rs:185:9
    |
 LL |         EMPTY_STR.is_empty();
    |         ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 27 previous errors
+error: aborting due to 28 previous errors