System.Linq: Avoid outputting to console in cast tests

Move to Assert.Throws() instead of catching the InvalidCastException so we
don't output to the console in Helper for some of the tests.

Author: akoeplinger

src/System.Linq/tests/LegacyTests/AverageTests.cs

@@ -778,41 +778,13 @@ public void Test()
 
         public class Average3f
         {
+            [Fact]
             // Type: long, OverflowException is thrown by param sum
-            public static int Test3f()
+            public void Test()
             {
                 long[] source = { Int64.MaxValue, Int64.MaxValue };
-                Console.WriteLine("111");
-                try
-                {
-                    var actual = source.Average();
-                    Console.WriteLine("222");
-                    return 1;
-                }
-                catch (OverflowException)
-                {
-                    Console.WriteLine("333");
-                    return 0;
-                }
 
-                catch (Exception e)
-                {
-                    Console.WriteLine("4444");
-                    Console.WriteLine(e);
-                    return 0;
-                }
-            }
-
-
-            public static int Main()
-            {
-                return Test3f();
-            }
-
-            [Fact]
-            public void Test()
-            {
-                Assert.Equal(0, Main());
+                Assert.Throws<OverflowException>(() => source.Average());
             }
         }