Reduce disk footprint of brin regression test

Per complaint from Tom.

While at it, throw in some extra tests for nulls as well, and make sure
that the set of data we insert on the second round is not identical to
the first one.  Both measures are intended to improve coverage of the
test.

Also uncomment the ON COMMIT DROP clause on the CREATE TEMP TABLE
commands.  This doesn't have any effect for someone examining the
regression database after the tests are done, but it reduces clutter for
those that execute the script directly.