Move test_size, bind_fid and wait_for_completion to common code.

Move wait_for_completion to common code and change simple/pingpong.c to
work with wait_for_completion.
I think currently size_option is not really making any selections here. How do you think of having a default set of safe transfer sizes (e.g., when '-S' is omitted), and have "all" to include all the rest of the sizes?