Command-buffer immediate command-list CTS fix

As discovered in
#1951 (comment)
there is an issue running the command-buffer CTS tests on L0 with
command-lists enabled.

After investigating, this was because the CTS tests we're not using the
output event parameter to command-buffer enqueue. In the L0 adapter
code a path for registering an event with the queue to waiting on
the workload was guarded by this event being set. Therefore
`urQueueFinish` was not working as expected, as the queue was not aware
of this work to wait on.

Fixed by always registering the work to wait on with the queue, and
miss out on propagation of the created event when user doesn't pass
an output event, rather than not creating it at all.

Author: EwanC

source/adapters/level_zero/command_buffer.cpp

@@ -1300,13 +1300,14 @@ ur_result_t waitForDependencies(ur_exp_command_buffer_handle_t CommandBuffer,
  * @param[in] CommandBuffer The command buffer.
  * @param[in] Queue The UR queue used to submit the command buffer.
  * @param[in] SignalCommandList The command-list to append the barrier to.
- * @param[out] Event The host visible event which will be returned to the user.
+ * @param[out][optional] Event The host visible event which will be returned
+ * to the user, if user passed an output parameter to the UR API.
  * @return UR_RESULT_SUCCESS or an error code on failure
  */
 ur_result_t createUserEvent(ur_exp_command_buffer_handle_t CommandBuffer,
                             ur_queue_handle_legacy_t Queue,
                             ur_command_list_ptr_t SignalCommandList,
-                            ur_event_handle_t &Event) {
+                            ur_event_handle_t *Event) {
   // Execution event for this enqueue of the UR command-buffer
   ur_event_handle_t RetEvent{};
 
@@ -1342,7 +1343,9 @@ ur_result_t createUserEvent(ur_exp_command_buffer_handle_t CommandBuffer,
                 &(CommandBuffer->SignalEvent->ZeEvent)));
   }
 
-  Event = RetEvent;
+  if (Event) {
+    *Event = RetEvent;
+  }
 
   return UR_RESULT_SUCCESS;
 }
@@ -1405,9 +1408,9 @@ UR_APIEXPORT ur_result_t UR_APICALL urCommandBufferEnqueueExp(
   ZE2UR_CALL(zeCommandListAppendEventReset,
              (SignalCommandList->first, CommandBuffer->AllResetEvent->ZeEvent));
 
-  if (Event) {
-    UR_CALL(createUserEvent(CommandBuffer, Queue, SignalCommandList, *Event));
-  }
+  // Appends a wait on the main command-list signal and registers output Event
+  // parameter with signal command-list completing.
+  UR_CALL(createUserEvent(CommandBuffer, Queue, SignalCommandList, Event));
 
   UR_CALL(Queue->executeCommandList(SignalCommandList, false, false));