Add test for pattern_type_mismatch.

This issue has been fixed by [commit](8c1c763)
This PR is used for close #7946(Fixes #7946).

changelog: Add test for pattern_type_mismatch.

Author: surechen

tests/ui/pattern_type_mismatch/mutability.rs

@@ -15,6 +15,15 @@ fn should_lint() {
         Some(_) => (),
         _ => (),
     }
+
+    const FOO: &str = "foo";
+
+    fn foo(s: &str) -> i32 {
+        match s {
+            FOO => 1,
+            _ => 0,
+        }
+    }
 }
 
 fn should_not_lint() {