[abigen][banjo] Tests for attribute markup

Confirm that the custom abigen attributes are handled as expected when
outputting .abigen from banjo.

(Also fixes unnecessary space in array spec that became apparent from
looking at test output.)

ZX-3882 #comment [abigen][banjo] Tests for attribute markup

Test: new codegen test
Change-Id: Id51fbaf0a4c8286af3ef8079dba2518a900430e9