Add cl_khr_command_buffer test that is it valid to release a
command-buffer after it has been enqueued but before execution is
finished.
This stresses the semantics from
[clReleaseCommandBufferKHR](https://registry.khronos.org/OpenCL/sdk/3.0/docs/man/html/clReleaseCommandBufferKHR.html#_description)
that: "After the command_buffer reference count becomes zero **and has
finished execution**, the command-buffer is deleted"