add net9.0

Author: kasperk81

tests/ImageSharp.Tests/ImageSharp.Tests.csproj

@@ -2,15 +2,27 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net8.0</TargetFrameworks>
     <DebugSymbols>True</DebugSymbols>
     <AssemblyName>SixLabors.ImageSharp.Tests</AssemblyName>
     <Platforms>AnyCPU;x64;x86;ARM64</Platforms>
     <RootNamespace>SixLabors.ImageSharp.Tests</RootNamespace>
     <Configurations>Debug;Release</Configurations>
   </PropertyGroup>
 
-  <ItemGroup>
+  <Choose>
+    <When Condition="$(SIXLABORS_TESTING_PREVIEW) == true">
+      <PropertyGroup>
+        <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
+      </PropertyGroup>
+    </When>
+    <Otherwise>
+      <PropertyGroup>
+        <TargetFrameworks>net8.0</TargetFrameworks>
+      </PropertyGroup>
+    </Otherwise>
+  </Choose>
+
+<ItemGroup>
     <InternalsVisibleTo Include="ImageSharp.Tests.ProfilingSandbox" Key="$(SixLaborsPublicKey)" />
   </ItemGroup>