Add test that AppContext.BaseDirectory is rooted (#113232)

Author: MichalStrehovsky

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/AppContext/AppContextTests.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.IO;
 using Xunit;
 
 namespace System.Tests
@@ -28,5 +29,11 @@ public void AppContext_ThrowTest()
         {
             AssertExtensions.Throws<ArgumentNullException>("name", () => AppContext.SetData(null, 123));
         }
+
+        [Fact]
+        public void BaseDirectory_PathRooted()
+        {
+            Assert.True(Path.IsPathRooted(AppContext.BaseDirectory), "BaseDirectory should be a rooted path");
+        }
     }
 }