Fix 051_effective_wal_level.pl on builds without injection points.

Fix 051_effective_wal_level.pl on builds without injection points.

Commit 2af1dc89282 placed the new "logical decoding disabled after
REPACK (CONCURRENTLY)" check at the end of
051_effective_wal_level.pl. That placement assumed the logical slot
"test_slot" no longer existed when the check ran, but the assumption
only holds on builds with injection points: the earlier
injection-point-driven tests drop "test_slot" as a side effect, while
on builds without injection points the slot persists.  When
"test_slot" still exists, logical decoding remains enabled and the new
check fails on those buildfarm members.

Move the REPACK test earlier in the script, ensuring that the test
starts with logical decoding disabled.

Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/CAD21AoBmdmBQ-+Jga+jSKKq5OPGEP1pEjSJfRPT6MCwVHLD6og@mail.gmail.com