[UR] Add known HIP failure for flaky test. (#18723)

Author: aarongreig

unified-runtime/test/conformance/exp_command_buffer/update/enqueue_update.cpp

@@ -184,6 +184,9 @@ TEST_P(urUpdatableEnqueueCommandBufferExpTest, SerializeAcrossQueues) {
 // Tests that submitting a command-buffer twice to an out-of-order queue
 // relying on implicit serialization semantics for dependencies.
 TEST_P(urUpdatableEnqueueCommandBufferExpTest, SerializeOutofOrderQueue) {
+  // See https://github.com/intel/llvm/issues/18722
+  UUR_KNOWN_FAILURE_ON(uur::HIP{});
+
   // First submission to out-of-order queue
   ASSERT_SUCCESS(urEnqueueCommandBufferExp(
       out_of_order_queue, updatable_cmd_buf_handle, 0, nullptr, nullptr));