Add a header command to dicttool.

This will allow to greatly improve the performance of the
metadata-generating files, as they won't have to wait for
the info command to read the entire dictionary when the
header is all we need.

Also add tests, and while we're at it, use the seed as
intended to enable reproducible tests.

Change-Id: I0ba79ef62f0292b23e63aed57ff565bb102281a2