add int return type for template functions

2020/DayTemplate.cs

@@ -9,16 +9,18 @@ namespace AdventOfCode {
 
   public class DayXX {
 
-    static void Part1() {
+    static int Part1() {
+      return -1;
     }
 
-    static void Part2() {
+    static int Part2() {
+      return -1;
     }
 
-    [Fact(Skip = "template")]
+    [Fact]
     public static void Test() {
-      Part1();
-      Part2();
+      Assert.Equal(-1, Part1());
+      Assert.Equal(-1, Part2());
     }
   }
 }