Restore missing consolidated nullable jit64 test projects

[Copilot]

PR #120992 converted nullable jit64 tests for consolidation but missed the consolidating project files for box-unbox and castclass, leaving 343 tests unbuilt and unexecuted. This change restores project ownership for those test trees by adding explicit aggregate .csproj files.

• What changed

  • Added /home/runner/work/runtime/runtime/src/tests/JIT/jit64/valuetypes/nullable/box-unbox/box-unbox.csproj
  • Added /home/runner/work/runtime/runtime/src/tests/JIT/jit64/valuetypes/nullable/castclass/castclass.csproj

• Project contents

  • box-unbox.csproj: includes all 193 subtree test sources
  • castclass.csproj: includes all 150 subtree test sources
  • Both projects also include shared /home/runner/work/runtime/runtime/src/tests/JIT/jit64/valuetypes/nullable/structdef.cs
  • Both reference $(TestLibraryProjectPath)

• Resulting behavior

  • The previously orphaned nullable jit64 tests are back under buildable test projects and can flow through normal test build/execution paths again.

<ItemGroup>
  <Compile Include="../structdef.cs" />
  <Compile Include="castclass/castclass001.cs" />
  <!-- ... -->
</ItemGroup>
<ItemGroup>
  <ProjectReference Include="$(TestLibraryProjectPath)" />
</ItemGroup>