Revise test case added in 43746996399541ecb5c7b188725a5f097c15ceae.

Revise test case added in 43746996399541ecb5c7b188725a5f097c15ceae.

Instead of using command_ok() to run psql, use safe_psql(). wrasse
isn't happy, and it be because of failure to pass -X to the psql
invocation, which safe_psql() will do automatically.

Since safe_psql() returns standard output instead of writing it to
a file, this requires some changes to the incantation for running
'diff'.

Test against the 'regression' database rather than 'postgres' so
we test more than just one table. That also means we need to record
the horizons later, after the test does "VACUUM FULL pg_largeobject".

Add an ORDER BY clause to the horizon query for stability.

Patch by me, reviewed by Tom Lane.

Discussion: http://postgr.es/m/CA+TgmoaGBbpzgu3=du1f9zDUbkfycO0y=_uWrLFy=KKEqXWeLQ@mail.gmail.com