tests: verify parsing logic for smart packets

The commits following this commit are about to introduce quite a lot of
refactoring and tightening of the smart packet parser. Unfortunately, we
do not yet have any tests despite our online tests that verify that our
parser does not regress upon changes. This is doubly unfortunate as our
online tests aren't executed by default.

Add new tests that exercise the smart parsing logic directly by
executing `git_pkt_parse_line`.