[llvm] [libsycl] add UT for kernel submission (PR #203931)

[Kseniya Tikhomirova via llvm-commits]

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@@ -430,6 +431,7 @@ class _LIBSYCL_EXPORT queue {
   std::shared_ptr<detail::QueueImpl> impl;
 
   friend sycl::detail::ImplUtils;
+  friend sycl::detail::MockQueue;
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KseniyaTikhomirova wrote:

@dvrogozh kindly ping, can this comment be resolved? 

https://github.com/llvm/llvm-project/pull/203931