Fix location of output logs of pg_regress

initdb.log and postmaster.log were moved to within the temporary instance
path by commit dcae5fac. This directory now gets removed at the end
of the run of pg_regress when there are no failures found, which makes
analysis of after-run issues difficult in some cases, and reduces the
output verbosity of the buildfarm after a run.

Fix by Michael Paquier

Backpatch to 9.5