Add a test for regular wildcard

fix

tests/ui/match_wildcard_for_single_variants.fixed

@@ -146,5 +146,11 @@ mod issue9993 {
             Foo::A(false) => 2,
             Foo::B => 3,
         };
+
+        let _ = match Foo::B {
+            _ if false => 0,
+            Foo::A(_) => 1,
+            Foo::B => 2,
+        };
     }
 }

tests/ui/match_wildcard_for_single_variants.rs

@@ -146,5 +146,11 @@ mod issue9993 {
             Foo::A(false) => 2,
             Foo::B => 3,
         };
+
+        let _ = match Foo::B {
+            _ if false => 0,
+            Foo::A(_) => 1,
+            _ => 2,
+        };
     }
 }

tests/ui/match_wildcard_for_single_variants.stderr

@@ -48,5 +48,11 @@ error: wildcard matches only a single variant and will also match any future add
 LL |         _ => (),
    |         ^ help: try this: `Color::Blue`
 
-error: aborting due to 8 previous errors
+error: wildcard matches only a single variant and will also match any future added variants
+  --> $DIR/match_wildcard_for_single_variants.rs:153:13
+   |
+LL |             _ => 2,
+   |             ^ help: try this: `Foo::B`
+
+error: aborting due to 9 previous errors