some fixes we've been carrying in our CTS fork:
* fix UB in `command_buffer_event_sync.cpp`: enqueue of two commands in
two separate queues, with both using the same buffer argument, and no
synchronization between the commands.
* fix UB in `command_buffer_test_barrier.cpp`: missing synchronization
between zeroing command and command-buffer using two separate queues
* make `test_thread_dimensions.cpp` thread-safe to avoid spurious
errors.