test: serialize Launcher.Tests via an assembly attribute, not xunit.runner.json

MainWindowViewTests kept failing on CI in Test Case Cleanup ('The calling
thread cannot access this object') while passing 56/56 locally. The cause was
delivery, not the fix: xunit.runner.json only takes effect if it is copied
beside the test DLL, and under CI's 'dotnet build' + 'dotnet test --no-build'
split it did not arrive, so CI ran with parallel collections while local runs
did not.

[assembly: CollectionBehavior(DisableTestParallelization = true)] is compiled
into the DLL and cannot fail to deploy. It lives beside the existing
AvaloniaTestApplication/AvaloniaTestIsolation attributes, which document the
same thread-affinity hazard. The json and its csproj copy rule are removed so
there is one source of truth.

tests/AcDream.Launcher.Tests/AcDream.Launcher.Tests.csproj

@@ -23,14 +23,4 @@
   <ItemGroup>
     <ProjectReference Include="..\..\src\AcDream.Launcher\AcDream.Launcher.csproj" />
   </ItemGroup>
-  <ItemGroup>
-    <!-- Avalonia's headless session is thread-affine, and its per-test cleanup
-         runs on whichever thread xUnit hands it. Under the default parallel
-         collections MainWindowViewTests failed in Test Case Cleanup with
-         "The calling thread cannot access this object because a different
-         thread owns it" while passing in isolation. Same serialization
-         settings AcDream.Core.Tests already uses. -->
-    <None Update="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
-  </ItemGroup>
-
 </Project>