Improve flaky test logs (#2246)

* Improve flaky test logs

* Fix another flaky

* Docs changes

* Add missing value

---------

Co-authored-by: GitHub Action <action@github.com>

README.md

@@ -87,7 +87,7 @@ await page.GoToAsync("http://www.google.com"); // In case of fonts being loaded
 await page.EvaluateExpressionHandleAsync("document.fonts.ready"); // Wait for fonts to be loaded. Omitting this might result in no text rendered in pdf.
 await page.PdfAsync(outputFile);
 ```
-<sup><a href='/lib/PuppeteerSharp.Tests/PageTests/PdfTests.cs#L29-L39' title='Snippet source file'>snippet source</a> | <a href='#snippet-pdfasync' title='Start of snippet'>anchor</a></sup>
+<sup><a href='/lib/PuppeteerSharp.Tests/PageTests/PdfTests.cs#L30-L40' title='Snippet source file'>snippet source</a> | <a href='#snippet-pdfasync' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->
 
 ### Inject HTML

lib/PuppeteerSharp.Tests/PageTests/ExposeFunctionTests.cs

@@ -140,15 +140,17 @@ public async Task ShouldKeepTheCallbackClean()
 
             // For CI/CD debugging purposes
             var session = (CDPSession)Page.Client;
+            var message = "Expected an empty callback list. Found: \n";
+
             if (session.HasPendingCallbacks())
             {
                 foreach(var pendingMessage in session.GetPendingMessages())
                 {
-                    Console.WriteLine($"Pending message: {pendingMessage.Message}");
+                    message += $" - {pendingMessage.Message}\n";
                 }
             }
 
-            Assert.False(((CDPSession)Page.Client).HasPendingCallbacks());
+            Assert.False(((CDPSession)Page.Client).HasPendingCallbacks(), message);
         }
     }
 }

lib/PuppeteerSharp.Tests/PageTests/PdfTests.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System;
+using System.IO;
 using System.Threading.Tasks;
 using Newtonsoft.Json;
 using PuppeteerSharp.Media;
@@ -75,7 +76,10 @@ public async Task CanPrintToPDFAndStreamTheResult()
             await Page.PdfAsync(outputFile);
 
             var stream = await Page.PdfStreamAsync();
-            Assert.Equal(new FileInfo(outputFile).Length, stream.Length);
+
+            // Firefox in Linux might generate and of by one result here.
+            // If the difference is less than 2 bytes is good
+            Assert.True(Math.Abs(new FileInfo(outputFile).Length - stream.Length) < 2);
         }
 
         [PuppeteerFact]

lib/PuppeteerSharp/CDPSession.cs

@@ -76,6 +76,7 @@ public async Task<JObject> SendAsync(string method, object args = null, bool wai
                 {
                     TaskWrapper = new TaskCompletionSource<JObject>(TaskCreationOptions.RunContinuationsAsynchronously),
                     Method = method,
+                    Message = message,
                 };
                 _callbacks[id] = callback;
             }