Adds tests to cover points 2 & 3 from the questions asked about
cl_khr_command_buffer_mutable_dispatch in
https://github.com/KhronosGroup/OpenCL-Docs/issues/1437
* New test for point 2 from issue, `mutable_dispatch_updates_persist`,
testing multiple enqueues of a command-buffer after update, and that the
updated argument persists for all of them.
* New test for point 3 pseudocode from issue in test
`mutable_dispatch_set_kernel_arg`