Add tests

Author: cston

src/Compilers/CSharp/Test/Symbol/Symbols/GenericConstraintTests.cs

@@ -1041,7 +1041,7 @@ static void M()
         }
 
         [WorkItem(528571, "DevDiv")]
-        [Fact(Skip = "528571")]
+        [Fact]
         public void ConstraintsWithinStruct()
         {
             var source =

src/Scripting/CSharpTest/InteractiveSessionTests.cs

@@ -394,6 +394,18 @@ public void Fields_Visibility()
             Assert.Equal(16, script.ContinueWith<int>("pi").EvaluateAsync().Result);
         }
 
+        [WorkItem(100639)]
+        [Fact]
+        public void ExternDestructor()
+        {
+            var script = CSharpScript.Create(
+@"class C
+{
+    extern ~C();
+}");
+            Assert.Null(script.EvaluateAsync().Result);
+        }
+
         #endregion
 
         #region Chaining