trailer: add unit tests for trailer iterator

trailer: add unit tests for trailer iterator

Test the number of trailers found by the iterator (to be more precise,
the parsing mechanism which the iterator just walks over) when given
some arbitrary log message.

We test the iterator because it is a public interface function exposed
by the trailer API (we generally don't want to test internal
implementation details which are, unlike the API, subject to drastic
changes).

Signed-off-by: Linus Arver <linus@ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>