Fix more portability issues in new pgbench TAP tests.

Fix more portability issues in new pgbench TAP tests.

* Remove no-such-user test case, output isn't stable, and we really
don't need to be testing such cases here anyway.

* Fix the process exit code test logic to match PostgresNode::psql
(but I didn't bother with looking at the "core" flag).

* Give up on inf/nan tests.

Per buildfarm.