Add ui_test error annotations in existing tests

Author: samueltardieu

tests/ui/ignored_unit_patterns.fixed

@@ -7,9 +7,11 @@ fn foo() -> Result<(), ()> {
 
 fn main() {
     match foo() {
-        Ok(()) => {},
-        Err(()) => {},
+        Ok(()) => {},  //~ ERROR: matching over `()` is more explicit
+        Err(()) => {}, //~ ERROR: matching over `()` is more explicit
     }
     if let Ok(()) = foo() {}
+    //~^ ERROR: matching over `()` is more explicit
     let _ = foo().map_err(|()| todo!());
+    //~^ ERROR: matching over `()` is more explicit
 }

tests/ui/ignored_unit_patterns.rs

@@ -7,9 +7,11 @@ fn foo() -> Result<(), ()> {
 
 fn main() {
     match foo() {
-        Ok(_) => {},
-        Err(_) => {},
+        Ok(_) => {},  //~ ERROR: matching over `()` is more explicit
+        Err(_) => {}, //~ ERROR: matching over `()` is more explicit
     }
     if let Ok(_) = foo() {}
+    //~^ ERROR: matching over `()` is more explicit
     let _ = foo().map_err(|_| todo!());
+    //~^ ERROR: matching over `()` is more explicit
 }

tests/ui/ignored_unit_patterns.stderr

@@ -19,7 +19,7 @@ LL |     if let Ok(_) = foo() {}
    |               ^ help: use `()` instead of `_`: `()`
 
 error: matching over `()` is more explicit
-  --> $DIR/ignored_unit_patterns.rs:14:28
+  --> $DIR/ignored_unit_patterns.rs:15:28
    |
 LL |     let _ = foo().map_err(|_| todo!());
    |                            ^ help: use `()` instead of `_`: `()`