Add environment variable controlled skipable LocalFactAttribute (#6009)

* Add environment variable controlled skipable LocalFactAttribute

* Fix copyright header

src/contrib/testkits/Akka.TestKit.Xunit2/Attributes/LocalFactAttribute.cs

@@ -0,0 +1,51 @@
+//-----------------------------------------------------------------------
+// <copyright file="LocalFactAttribute.cs" company="Akka.NET Project">
+//     Copyright (C) 2013-2022 .NET Foundation <https://github.com/akkadotnet/akka.net>
+// </copyright>
+//-----------------------------------------------------------------------
+
+using System;
+using Xunit;
+
+namespace Akka.TestKit.Xunit2.Attributes
+{
+    /// <summary>
+    /// <para>
+    /// This custom XUnit Fact attribute will skip unit tests if the environment variable
+    /// "XUNIT_SKIP_LOCAL_FACTS" exists and is set to the string "true"
+    /// </para>
+    /// <para>
+    /// Note that the original <see cref="FactAttribute.Skip"/> property takes precedence over this attribute,
+    /// any unit tests with <see cref="LocalFactAttribute"/> with its <see cref="FactAttribute.Skip"/> property
+    /// set will always be skipped, regardless of the environment variable content.
+    /// </para>
+    /// </summary>
+    public class LocalFactAttribute: FactAttribute
+    {
+        private const string EnvironmentVariableName = "XUNIT_SKIP_LOCAL_FACTS";
+
+        private string _skip;
+
+        /// <inheritdoc cref="FactAttribute.Skip"/>
+        public override string Skip
+        {
+            get
+            {
+                if (_skip != null)
+                    return _skip;
+
+                var skipLocal = Environment.GetEnvironmentVariable(EnvironmentVariableName)?
+                    .ToLowerInvariant();
+                return skipLocal is "true" ? SkipLocal ?? "Local facts are being skipped" : null;
+            }
+            set => _skip = value;
+        }
+
+        /// <summary>
+        /// The reason why this unit test is being skipped by the <see cref="LocalFactAttribute"/>.
+        /// Note that the original <see cref="FactAttribute.Skip"/> property takes precedence over this message. 
+        /// </summary>
+        public string SkipLocal { get; set; }
+    }    
+}
+