aga: Add lazytrie testcase

This adds a more complex testcase to the aga module.  This one is a trie
(basically a radix tree for strings).

It demonstrates different ways of constructing edge information from an
internal representation than the existing testcases.  Importantly, it also
demonstrates aga's ability to cope with the edge function lazily
constructing nodes on the fly.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>