Test fixes

src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

@@ -169,7 +169,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff
             long currentOffset = writer.BaseStream.Position;
             foreach ((long, uint) marker in this.frameMarkers)
             {
-                writer.WriteMarker(marker.Item1, marker.Item2);
+                writer.WriteMarkerFast(marker.Item1, marker.Item2);
             }
 
             writer.BaseStream.Seek(currentOffset, SeekOrigin.Begin);

src/ImageSharp/Formats/Tiff/Writers/TiffStreamWriter.cs

@@ -125,6 +125,14 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Writers
         /// <param name="offset">The offset returned when placing the marker</param>
         /// <param name="value">The four-byte unsigned integer to write.</param>
         public void WriteMarker(long offset, uint value)
+        {
+            long back = this.BaseStream.Position;
+            this.BaseStream.Seek(offset, SeekOrigin.Begin);
+            this.Write(value);
+            this.BaseStream.Seek(back, SeekOrigin.Begin);
+        }
+
+        public void WriteMarkerFast(long offset, uint value)
         {
             this.BaseStream.Seek(offset, SeekOrigin.Begin);
             this.Write(value);