Make PostgreSQL::Test::Cluster::run_log() return a useful value.

Make PostgreSQL::Test::Cluster::run_log() return a useful value.

Curently, some TAP test that directly call the underlying function
PostgreSQL::Test::Utils::run_log() care about the return value, but
none of those that call it via PostgreSQL::Test::Cluster::run_log() care.
However, I'd like to add a test that will care, so adjust this function
to return whatever it gets back from the underlying function, just as
we do for a number of other functions in this module.

Discussion: http://postgr.es/m/CA+Tgmobj6u-nWF-j=FemygUhobhryLxf9h-wJN7W-2rSsseHNA@mail.gmail.com