brin regression test intermittent failures

There's something odd about the brin regression tests. They seem to 
generate intermittent failures, which suggests some sort of race 
condition or ordering failure.

See for example 
<http://www.pgbuildfarm.org/cgi-bin/show_log.pl?nm=fulmar&dt=2015-05-15%2001%3A02%3A28>
and 
<http://www.pgbuildfarm.org/cgi-bin/show_log.pl?nm=sittella&dt=2015-05-15%2021%3A08%3A38>

cheers

andrew