Add page.screenshot viewport clipping test (#1320)

* Add page.screenshot viewport clipping test

* Refactor

Author: kblok

lib/PuppeteerSharp.Tests/PageTests/ScreenshotTests.cs

@@ -90,6 +90,28 @@ await page.SetViewportAsync(new ViewPortOptions
             }
         }
 
+        [Fact]
+        public async Task ShouldClipElementsToTheViewport()
+        {
+            using (var page = await Context.NewPageAsync())
+            {
+                await page.SetViewportAsync(new ViewPortOptions { Width = 500, Height = 500 });
+                await page.GoToAsync(TestConstants.ServerUrl + "/grid.html");
+                var screenshot = await page.ScreenshotDataAsync(new ScreenshotOptions
+                {
+                    Clip = new Clip
+                    {
+
+                        X = 50,
+                        Y = 600,
+                        Width = 100,
+                        Height = 100
+                    }
+                });
+                Assert.True(ScreenshotHelper.PixelMatch("screenshot-offscreen-clip.png", screenshot));
+            }
+        }
+
         [Fact]
         public async Task ShouldRunInParallel()
         {
@@ -148,12 +170,12 @@ public async Task ShouldRunInParallelInMultiplePages()
             var pageTasks = new List<Task<Page>>();
             for (var i = 0; i < n; i++)
             {
-                Func<Task<Page>> func = async () =>
+                async Task<Page> func()
                 {
                     var page = await Context.NewPageAsync();
                     await page.GoToAsync(TestConstants.ServerUrl + "/grid.html");
                     return page;
-                };
+                }
 
                 pageTasks.Add(func());
             }