[SYCL] Fix use-after-free for MockHandler::getKernelName() (#18801)

In current implementation, std::string is returned from getKernelName().
For preview, caller of getKernelName() creates std::string_view from the
returned temporary string. This sting_view is used after the temporary
string is destroyed. In the fix for preview std::string_view is
returned, that points to same kernel name as handler::MKernel.

Author: Alexandr-Konovalov

sycl/unittests/scheduler/SchedulerTestUtils.hpp

@@ -253,7 +253,9 @@ class MockHandler : public sycl::handler {
     return impl->CGData.MEvents;
   }
   std::vector<sycl::detail::ArgDesc> &getArgs() { return impl->MArgs; }
-  std::string getKernelName() { return MKernelName.data(); }
+  sycl::detail::KernelNameStrT getKernelName() {
+    return toKernelNameStrT(MKernelName);
+  }
   std::shared_ptr<sycl::detail::kernel_impl> &getKernel() { return MKernel; }
   std::shared_ptr<sycl::detail::HostTask> &getHostTask() {
     return impl->MHostTask;