test: add match sumtype var shadow and `as` test

vlib/v/tests/match_sumtype_var_shadow_and_as.v

@@ -0,0 +1,31 @@
+struct Cat{name string}
+struct Dog{name string}
+
+type Animal = Cat | Dog
+
+fn main() {
+	cat := Cat{name: 'cat'}
+	dog := Cat{name: 'dog'}
+	mut animal := Animal{}
+	
+	// test shaddow
+	animal = cat
+	match animal {
+		Cat {
+			assert animal.name == cat.name
+		}
+		else{
+			assert false
+		}
+	}
+	// test as
+	animal = dog
+	match animal as animal_kind {
+		Dog {
+			assert animal_kind.name == dog.name
+		}
+		else{
+			assert false
+		}
+	}
+}