pgbench aggregates final part can be missed at log

Hi hackers!

Recently Kirill and I did some benchmarks with --log
and --aggregate-interval and noticed inaccuracy:
transactions that fall into the last, not-yet-closed interval are
silently omitted from the log. doLog() only emits intervals that have
fully elapsed, and the flush done at the end of a run walks the same
fully-elapsed intervals - so the tail is dropped without any warning.

Deterministic reproducer:
$ pgbench -i -s 1 pq
$ pgbench -c 1 -t 60 -n -l --aggregate-interval=5 -f onetx.sql pq
number of transactions actually processed: 60/60
$ cat pgbench_log.* # empty!

POC-patch of the accounting fix attached, it does:
- the interval-printing code is split into printAggInterval() (print
one row), flushElapsedAggIntervals() (loop over completed intervals,
shared by doLog() and the shutdown flush)
- at shutdown, flushElapsedAggIntervals() first flushes the fully
elapsed intervals, then the final, possibly partial interval is
written as well, when it collected any data (aggIntervalHasData())
- a new TAP test asserts that a run much shorter than the interval
logs exactly one row accounting for all processed transactions

Any feedback would be appreciated!

--
Best regards,
Roman Khapov