Add references to xunit.*.dll to CLRTest.Crossgen.targets (#83413)

Add references to xunit.*.dll to CLRTest.Crossgen.targets (#83413)

This change fixes the exceptions thrown from Crossgen2 JIT
interface when JIT tries to resolve tokens from xunit.assert.
As a secondary effect the frequent exception throws triggered
the race condition

https://github.com/dotnet/runtime/issues/81884

triggering non-deterministic native CoreCLR runtime failures on
arm64. I was originally reluctant to fix this primary issue until
the native runtime bug is understood as otherwise this fix would
just cause the issue to stop manifesting without being actually
fixed. As the runtime bug has been fixed by now, I am fixing
Crossgen2 executions to stop hitting these resolution exceptions.

Thanks

Tomas