Make FileSystemStream properties virtual: (#1099)

- IsAsync
- Name

Author: vbreuss

src/TestableIO.System.IO.Abstractions/FileSystemStream.cs

@@ -27,14 +27,14 @@ public override bool CanWrite
             => _stream.CanWrite;
 
         /// <inheritdoc cref="FileStream.IsAsync" />
-        public bool IsAsync { get; }
+        public virtual bool IsAsync { get; }
 
         /// <inheritdoc cref="Stream.Length" />
         public override long Length
             => _stream.Length;
 
         /// <inheritdoc cref="FileStream.Name" />
-        public string Name { get; }
+        public virtual string Name { get; }
 
         /// <inheritdoc cref="Stream.Position" />
         public override long Position