Factor out padding, use concise initialization syntax
Use target_word_size to conditionally set the padding size
UnicastThroughputTest: parametrize the number of iterations

This change adds a flag to UnicastThroughputTest to choose the number of
iterations to use. For casual testing after every source code change, a
number like 1 000 000 strikes a better balance between convenience and
validity. This will become more important as more tests are added.