Update test SDK

note: v15.7 continues to work on older VS versions

tests/ImageSharp.Tests/ImageSharp.Tests.csproj

@@ -33,22 +33,16 @@
     <PackageReference Include="xunit.runner.console" Version="2.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
     <PackageReference Include="Moq" Version="4.7.145" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.7.0" />
     <PackageReference Include="xunit" Version="2.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
     <PackageReference Include="Moq" Version="4.8.1" />
-    <!--<PackageReference Include="StyleCop.Analyzers" Version="1.1.0-beta001">
-      <PrivateAssets>All</PrivateAssets>
-    </PackageReference>-->
   </ItemGroup>
   <ItemGroup>
     <AdditionalFiles Include="..\..\stylecop.json" />
     <ProjectReference Include="..\..\src\ImageSharp.Drawing\ImageSharp.Drawing.csproj" />
     <ProjectReference Include="..\..\src\ImageSharp\ImageSharp.csproj" />
   </ItemGroup>
-  <ItemGroup>
-    <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" />
-  </ItemGroup>
   <ItemGroup>
     <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.1" />
   </ItemGroup>