pgsql: Use command_ok for pg_regress calls in 002_pg_upgrade and 027_st

Use command_ok for pg_regress calls in 002_pg_upgrade and 027_stream_regress

Now that command_ok() captures and displays failure output, use it
instead of system() plus manual diff-dumping in these two tests. This
simplifies both scripts and produces consistent, truncated output on
failure.

Author: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Reviewed-by: Andrew Dunstan <andrew(at)dunslane(dot)net>
Reviewed-by: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Discussion: https://postgr.es/m/DFYFWM053WHS.10K8ZPJ605UFK@jeltef.nl

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bb6ae9707c1789c9d2036c9bbd35965a01e1b214

Modified Files
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src/bin/pg_upgrade/t/002_pg_upgrade.pl | 40 ++++++++--------------
src/test/recovery/t/027_stream_regress.pl | 55 +++++++++----------------------
2 files changed, 29 insertions(+), 66 deletions(-)