Fix headless

src/Avalonia.Headless/HeadlessPlatformStubs.cs

@@ -102,7 +102,7 @@ namespace Avalonia.Headless
 
         public int GlyphCount => 1337;
 
-        public FontSimulations FontSimulations => throw new NotImplementedException();
+        public FontSimulations FontSimulations { get; }
 
         public void Dispose()
         {

tests/Avalonia.Skia.UnitTests/Media/CustomFontManagerImpl.cs

@@ -103,7 +103,7 @@ namespace Avalonia.Skia.UnitTests.Media
                     }
             }
 
-            return new GlyphTypefaceImpl(skTypeface);
+            return new GlyphTypefaceImpl(skTypeface, FontSimulations.None);
         }
     }
 }