a few fixes (thread safety & cl_khr_command_buffer UB)

[franz]

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.

[franz]

Removed the first two commits from the PR; i did not realize there was already an open PR with those.

[franz]

rebased on main. should be ready for review.

[bashbaug]

Merging as discussed in the October 8th teleconference.