GIN predicate locking slows down valgrind isolationtests tremendously

Hi,

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2018-03-31%2014%3A40%3A02
isolation-check (02:19:36)

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2018-03-28%2010%3A40%3A02
isolation-check (00:18:44)

As far as I can tell that increase comes laregely from the new GIN
tests.  Could one of you please look at keeping the test time increase
to something more reasonable?

Greetings,

Andres Freund