chainlint: add explanatory comments

chainlint: add explanatory comments

The logic in TestParser::accumulate() for detecting broken &&-chains is
mostly well-commented, but a couple branches which were deemed obvious
and straightforward lack comments. In retrospect, though, these cases
may give future readers pause, so comment them, as well.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>