The thought of psql_help purely because it was the easiest at that time. Since I've just begun my understanding of the code is barely negligible.
I began working on SEQUENCE related tests thereafter and hopefully would move to more complicated tests in time. Peter's link is obviously helpful but since I end up doing make check ~100 of times a day, for now its useful only to cross-check how much code is uncommitted :)
I think increasing coverage is a good thing. But psql help? *shrug* backend code is far more interesting and useful.
Another thing to keep in mind is that there are some corner cases that are interesting to test that might not necessarily show up in a coverage chart -- for example how stuff behaves in the face of concurrent processes, or when various counters wrap around.
