Ah, I finally realize why Magnus wanted to add a --bindir option to

pg_regress: there's no other way to cope with testing a relocated
installation.  Seems better to call it --psqldir though, since the
only thing we need to find in that case is psql.  It'd be better if
we could use find_other_exec, but that's not happening unless we are
willing to install pg_regress alongside psql, which seems unlikely
to happen.