Add test for error message

Author: camelid

src/test/ui/slice-to-vec-comparison.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let a = &[];
+    let b: &Vec<u8> = &vec![];
+    a > b;
+    //~^ ERROR mismatched types
+}

src/test/ui/slice-to-vec-comparison.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/slice-to-vec-comparison.rs:4:9
+   |
+LL |     a > b;
+   |         ^ expected array of 0 elements , found struct `Vec`
+   |
+   = note: expected reference `&[_; 0]`
+              found reference `&Vec<u8>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.