tests: Updates and fixes for tests, changing master to main and adding flaky tests marker (#2511)

Octokit.Tests.Integration/PotentiallyFlakyTestAttribute.cs

@@ -0,0 +1,13 @@
+using System;
+
+namespace Octokit.Tests.Integration
+{
+    /// <summary>
+    /// A potentially flaky test could be:
+    /// * Calls for details of repositories which already exist and was not created by the test itself (so could disappear over time)
+    /// * Calls for details of users which already exist (so could disappear over time)
+    /// </summary>
+    public class PotentiallyFlakyTestAttribute : Attribute
+    {
+    }
+}