Add Wait to ToLevelLambda tst

src/Scripting/CSharpTest/InteractiveSessionTests.cs

@@ -823,7 +823,7 @@ public void TopLevelLambda()
 
             using (var redirect = new OutputRedirect(CultureInfo.InvariantCulture))
             {
-                s.ContinueWith(@"testDelB(""hello"");");
+                s.ContinueWith(@"testDelB(""hello"");").Wait();
                 Assert.Equal("hello", redirect.Output.Trim());
             }
         }