Java gen: fix alignment for action set_field, add java_gen unit tests for actions

Reviewer: @rlane @vaterlaus @yotamhc

This pull requests fixes the alignment of the class OFActionSetField. Actions must be aligned to 64 bits according to the spec, and the alignment is *included* in the length (vs. the MatchV3 class, where the alignment is not included in the length *sigh*). 

This PR adds support for declaring whether the length includes the alignment or not to the input files (param length_includes_align). It also changes the unit test lookup mechanism somewhat to make java_gen find the set_action-related unit tests.