Migrate test projects from xunit 2.9.3 to xunit.v3 3.2.2

Test projects become executables (OutputType=Exe) as required by v3.
Satisfy the new xUnit1051 analyzer by passing TestContext cancellation
tokens in the generator compilation test.

tests/MrGameEng.Assets.Generator.Tests/MrGameEng.Assets.Generator.Tests.csproj

@@ -2,12 +2,13 @@
 
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
+    <OutputType>Exe</OutputType>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
-    <PackageReference Include="xunit" />
+    <PackageReference Include="xunit.v3" />
     <PackageReference Include="xunit.runner.visualstudio" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" />
   </ItemGroup>